1988 Cadillac EL Dorado engine misses and stumbles

My engine runs normal about 98% of the time. After the car is driven for 30 minutes or so the engine sometimes misses, especially if the engine gets a little warm. I use the on board computer to monitor the temperature when this happens. Just a few degrees, say 10 degrees, just going up a grade is enough to make it do it. Seems to do it more on a hot day. I have put in a new thermostat. That did nothing to help. I have put in two oxygen sensors. That did nothing to help. This problem started about two thousand miles ago. It acts like a vapor lock to me. Is that possible? Nothing has been altered on the car, it is completely stock. I put a pressure gauge on the fuel line leading to the throttle body and taped the gauge to the windshield and drove the car for about an hour. The fuel pressure was about 9 PSA all the time, whether the engine misses and stumbles or not. The are no ECM codes set. .