I had this happen to me about 2 years. After running through every possibility over a period of a couple of weeks, I found that the battery ground cable (that connects to the bell housing) was charred pretty bad and wasn't getting a good ground. The car would start just fine when cold but, once it had been run for awhile, I would have to let it for 15-30 minutes before it would start.

The harness from Porsche is around $200. However, I picked up a military style terminal, a 36" and a 6" battery cable from Napa for around $100. Haven't had the problem since.