Chris,
It seems to me, your problem is more likely due to driving time than temperature and my first guess is the ignition coil. It's pretty typical of a bad coil to work OK for a certain period of time, then crap out as it gets hot. Do you happen to have an MSD CD ignition box? If so, that could also be the culprit. Like I said in a previous posting, when the car won't start, pull a plug wire and see if you have ignition.