90 Lumina TPS circuit problems.

Hi. I'd be extremely grateful if someone could point me in the right direction on a car I'm working on. It's a 1990 Lumina, 1st # in VIN is a 2. The car is very hard to start, but when it does, it will run like a top (at around 1/4 to 1/3 throttle for about 3 seconds), then the service engine soon light comes on, and dies. The code is 12, and 22. The TPS is new, and I even replaced it a 2nd time to verify I didn't get a bad part. Initially, I had continuity between the 5volt reference and the ground wires going to the TPS, which led me to believe I have a short in the TPS circuit. After R&R of the upper plenum, tracing down wires, etc, I have found no visible short. After reassembling everything, I no longer have continuous continuity between the 5 volt reference and the ground, but can occasionally get a 'spike' on my multi-meter. I cannot determine if there is an individual wire that the insulation has broken down in, or if the computer is bad. I don't believe the computer to be bad, because I have a spare computer that I swapped out, only to get the exact same symptoms that I have now. Any ideas? I'm stumped, and taking a break from it for now. Thanks for any and all help.

Update: I also forgot to mention that I am not getting 5 volts to the tps pigtail. I am getting approximately.98 volts to the pigtail, from the computer. I have checked the voltage at the computer, and at the tps connector, as well as at the MAP connector (which is shared between the two), and at all three locations the voltage is the same.