I hope you meant "and DROP 3-10 psi at idle"

Hey! It does say that doesn't it... oops.

This morning before work the car was still running bad after I started it. It slowly gets better as it warms up and seems to be running almost perfectly once I get to work since it'* warmed up by then. After work tonight, and sitting in 15*F for 16 hours, it was running bad again until it warmed up enough to start to drive it home. Even then it wasnt perfect. So we braved the cold and tested the fuel pressure...

Turn key on, 48psi.
Idle, 42psi
Shut engine off, 36psi.

This all seems decent but by now it was idling a little better since it was still warm. I may dig into the throttle body tomorrow, it'* gonna be a cold one again too, with a high of 11*F...

Also, when it'* idling rough and almost dying like it was both times today, the check engine light doesn't come on...

Okay, now I plugged it in last night hopeing it would be better this morning and it was. It didn't sputter or try to die at all. So I made it to work fine and all. Then when I was going to go home about an hour ago, went out to start it, back to the sputtering not wanting to run right.

How could keeping it plugged in cause it to run better than not being plugged in? Just cold oil and fluids???

Have you tried checking the coils? http://www.bonnevilleclub.com/forum/...e=article&k=33

I haven't tried the coils yet, it'* too damn cold out right now. I also noticed something, when the GM dealer changed my oil about 1000 miles ago, not that the mileage matters, they used 10W-30. I should have had them put in 5W-30, but didnt think about it. How much of a difference could oil weight make in a case like this? Running fine after being plugged in?

There'* a few things that get affected by temperature like the crankshaft sensor..which by itself is not expensive,but one hellava job to get to it.. a faulty CS..will make your engine stutter and also make it surge before and after shift changes.

Can this go bad? When I was changing the vacuum connections the check-valve had what looked like rust or grungy ATF in it. Kinda red, dirty stuff.
That is sounding a little like my problem. When I'm slowing down for a red light/stopsign/whatever the car seems to want to chug foward a little bit, so I usually just throw her in neutral while I'm stopped.

If you find trans fluid in the line to the vacuum modulator, it'* time for a new modulator!

Have you checked the CTS? It could be stuck at a value around 400 ohms causing the ECM to think the engine is always warm. Since it thinks its warm, it will go into closed loop and the O2 is not at temp yet. Just a thought.