FINALLY!!! After 6 months of trying to figure out what's been freaking out the Boxster, I gave up and took it to the local non-dealer P-car pro. It took them awhile, but she's back to normal again. :dance:
The problem started with a P1502 code (throttle jacking spring) after I replaced the turn signal switch. I finally figured out it would drive when the code was cleared after starting it every time while I tried to figure everything else out. Then, over time, the cruise stopped working and the windows wouldn't drop when the top latch was popped. After several parts were replaced and many hours of diagnostics, the car freaked out and would not accellerate at all, even when the code was cleared. That was my breaking point so I took her in.
The mechanic had her for two weeks and did several tests, including wire-by-wire diagnostics to try and figure it out. After checking everything out, he couldn't find a problem so he asked if the computer could be sent out for testing. I agreed, then he removed the computer and prepped it for shipping. He called several places to get it tested and everyone said it would be awhile before they could get to it. He left it in the shipping box for a few days and thought that may reset the computer so he removed it and put it back into the car. It ran perfectly!! Now it's running like a champ again, no problems at all and everything (including cruise & top) is working.
It felt sooooo nice to drive her home!!