I recently purchased a 1999 Buick Park Avenue Ultra w/ 3.8l 3800 Series II Supercharged V6 with 200,000 miles. When I purchased it, was throwing a P0302 code (Cylinder 2 misfire). The car was a good price and I (wrongfully) assumed the misfire was likely just a simple plug/wire related issue, so I bought it. Immediately after purchase, I:
-Changed the oil
-Swapped in new spark plugs
-Swapped in new plug wires
-Swapped in new coil packs
None of these repairs fixed the misfire, however.

The misfire itself comes and goes - one day the vehicle will drive fine with ndo issues, and the next, it will act up furiously and misfire very aggressively as I'm driving it. The other day, I had just started the vehicle up when I noticed it was acting very sluggish - as if it was having trouble pulling in air. I gave it a little gas and the engine stalled out at around 40mph, and wouldn't allow me to turn it back on afterwards. I didn't have my code reader but it was flashing the "Check Engine" light and dinging at me. I had been driving for less than a minute. I know a little bit about cars, but I am far from an expert, so I am not sure if these two issues are related? Additionally, the car will have difficulty starting up sometimes. Sometimes, it will grind as if the starter is going out, but again, other times, it works fine and starts up immediately.

I have performed the following maintenance on it attempting to solve the misfire/stall issue, but none of it has solved the problem.
-Replaced fuel injectors with refurbished injectors out of a 1997 Park Avenue W/O supercharger (cleaned them + new O-rings/pintle caps)
-Replaced throttle body gasket
-Replaced intake manifold gasket
-Replaced fuel pump
-Replaced ECM
-Tested Mass Air Sensor (it was fine)
-Tested Crank Position Sensor (it was also fine)
-Tested battery (it was fine, reading between 12.8 to 14.0 volts)

I am beginning to think that the problem must be within the wiring harness. Does anyone have any ideas where to start looking and if those issues mentioned above would all be related to a short? Or, does anyone have any other ideas before I start that job?

Thanks in advance.

If you don't mind me asking...what is a "good" price for a 20 year old Buick with 200k miles?

One immediate concern is that the supercharged application uses different injectors than the naturally aspirated application (higher flow rate).

I would be checking compression as the next step, because you have ruled out both spark and fuel related concerns on this one cylinder.

As a general rule, injector/compression problems don't go away.....so when you say it runs great one day and may be a dog the next, I tend to think it is either a wiring problem, ICM problem, crank or cam sensor problem, or PCM problem.....

No other code besides the P0302?

With the engine running, I would SAFELY, tug on any wire harness I could reach, to see if it affects the operation of the engine....

Check fuel pressure at idle. Disconnect vacuum hose from fuel pressure regulator. Does fuel pressure change? Let us know before and after.

Could be a flaky fuel-pressure regulator, although it would be weird for it to affect one cylinder.

I almost forgot.....have you check the PCM and the wires to it?