If you feel up to the job, you should go onto Dave Barton's website and order a wiring harness from him, he will mail it out.

http://www.linkline.com/personal/dbarton/

I doubt a local shop will put it in for you though. The ones he sells are used, although in excellant condition. So, you would have to put it in yourself. It might fix your current problem, but if it doesn't it will definately fix future problem before they happen and you are on the side of the road.

But as far as my diagnosis, I wasn't really suggesting that your car was overheating. Just that when you increase the temp of the engine and all surrounding components under the hood from room temp to standard operating temp, around 200F, things can change like resistances in wires are seals created by gaskets. And since the problem only occures after the car has been running for a while, it sounds like it might be due to everything getting warm. Other than that I don't know what changes from 5 min in vs. 20 min into a drive.
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- Mike