Interesting. I am wondering if the PCM may be the culprit or a connector/ground issue.
I am going to contact one of our ASE Certified Gear Heads & see if I can get his input into this thread. Hang in there..

The video that was shown, was everything that I could possibly think of....especially checking the wire harness(bad connection could distort the resistance/voltage readings of the TPS.......but if the harness wires and connectors are ok, and the throttle body has been cleaned, and the new throttle body comes up with the same numbers, then with the key out of the ignition, disconnect the neg bat cable first, and then the positive.....then hold together(I use a jumper cable) the two ends of the battery cable, for a minute or two....then attach pos cable , and then the neg cable....this will reset ALL modules.....

If that doesn't take care of the problem, then I think a dealership should check into a a reprogram, especially for the TAC system......make sure ALL modules have the latest updates.....

Also, did the GM Dealership do a TSB search for the TAC code?

One thing the video didn't show is terminal tension. The inside of the terminal couldn't be making good contact with the male end terminal.
Here is one bulletin I found
Subject: SES Light and Reduced Engine Power DTC P0120 P0220 P1516 P2135

Models: 2004-2006 Buick Rainer
2005-2006 Cadillac CTS-V
2003-2006 Cadillac Escalade
2003-2006 Chevrolet Avalanche, Corvette, Express, Monte Carlo, Silverado, SSR, Suburban, Tahoe, Trailblazer
2003-2006 GMC Envoy, Savana, Sierra, Yukon
2003-2006 Hummer H2
2005-2006 Pontiac GTO
Equipped with a 4.8 5.3 6.0 or 7.0 V-8 Engine

--------------------------------------------------------------------------------

The following diagnosis might be helpful if the vehicle exhibits the symptom(*) described in this PI.

Condition/Concern:
A vehicle may be brought into the dealer for a reduced power message, and DTCs P0120, P0220, P1516, P2101, or P2135.

The Throttle Actuator Control (TAC) / throttle body type trouble codes, may be caused by a loose wiring crimp at the throttle body connector, or a broken throttle body circuit.

Recommendation/Instructions:
Complete the current SI diagnostics for any symptoms or trouble codes found. If a intermittent T/P or TAC module type code is occurring complete the inspections below.

 1. Inspect all related throttle body terminals for a loose wiring crimp. The loose crimp may be difficult to find, and the poor connection will be between the terminal and the copper strands of the wire. Wiggle test the individual throttle body circuits to see if the concern can be duplicated.
 2. Inspect the related circuits for broken wires inside the insulation. The outer wire insulation may look fine, but the internal copper strands may be partially broken. Breaks in the wires usually occur within 1 to 4 inches of the throttle body connector. Wiggle testing may also induce a trouble code to set.
 3. On C/K trucks complete SI procedures for voltage drop on grounds G103 and G104. Grounds G103 or G104 may be loose or corroded.
If a terminal crimp or a broken wire has been found, repair or replace only the circuits involved. There is a throttle body pigtail connector available through GMSPO, but installing this pigtail connector may cause other intermittent TAC module/TP codes at a later date. If this pigtail must be used, please follow the SI procedures for Splicing Copper Wire Using Splice Sleeves. (the proper Kent-Moore crimping tool must be used for this repair)

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.

Latest update:

I got froggy and did a couple things. The shop traced a whine back to what they thought was the alternator bearing and/or idler pulley next to it. So I replaced both and got new belts (they needed it). The night of, the whine was still there, but today, all day, it was gone. Not entirely related to the situation, but it caused me to have the battery disconnected for a while (about four hours, to which some of the time, I left the leads jumpered for a while.) While I was in there I replaced the air intake with a straight tube conical filter type (I am happy with those results). The whole job only cost me about $300 to do myself (I know my was around some pretty hefty Advance Auto online coupon codes, and saved about $100 overall by splitting up the purchases, so the air intake upgrade only wound up costing me $110.)

Since then, even though my instrument cluster doesnt register anything until the engine is running, it has, at least been consistently been coming on at each start up.

Where would I find grounds G103 and G104? Is there something to be said for removing and physically cleaning the TAC? If I'm going to take a multimeter to the TPS or TAC connections, how much resistance should I expect between the terminal end and the engine block (common ground)?

Thanks everyone for the help on this matter!
-Mike

You don't have to remove the throttle body to clean...just hold it open, and with a rag soaked with carb cleaner, clean the edges of the plate and the bore....the plate will snap back into place....check for binding/sticking.....after cleaning the plate/bore, that is when you should do the bat cable reset....

There are no resistance values.....resistance at any connection should be zero.....

As far as the TSB is concerned, it'* similar to the video...the video shows the insulation intact, but the wire definitely broken at the crimp....where resistance can come into play, is the female connectors in the harness......you would need a male connector, to do a "drag test"(get this from GM parts dept.)....basically insert the male connector into each female connector in the harness....then pull it out....there should be equal "feel of resistance" when removing the male connector from the female connectors....if one feels loose, it should be replaced......this looseness, can cause arcing/corrosion, which could increase resistance in the circuit, and skew the position data....