Thanks for all the responses. Most of them seemed to point to electrical problems of some sort, and none seemed to confirm the clogged cat theory.

I talked to the mechanic who made the original diagnosis. He seemed to really know what he was talking about and convinced me he had ruled out the electrical problems. Even though he wouldn't guarantee 100% the new exhaust would fix the problem, I took a chance and let him replace the exhaust from the converter back.

That fixed the problem, and the car has more power than it seemed to have before. My son has been driving the car several months and had commented on the lack of power, but he didn't have anything to compare it to. I guess the exhaust was partially clogged the whole time and it got progressively worse.

The "start and run fine and then gradually deteriorate" symptoms were explained by the mechanic as not necessarily temperature related, but related to the continual build up of back pressure until the engine quit breathing.

Anyway, just a final note of thanks and report of the conclusion.

Robert