2001 Saturn SC2 Help

My problem started this way. The car had occasionl idle issues, afew times it would die while stoped at a red light. This problem was very intermittend. Then would not start occasionally, when it did run it would have idle problems, sometimes seemed like it wanted to die alltogether, regardles of what speed it was running at. So far I had the fuel filter, fuel pump and the crank shaft sensor replaced. My mechanic still can't get the fuel pressure to come up to par. The car will run fine while cold, once it heats up it starts loosing power. Which hapens immideately if I turn on the AC. Any suggestions will be appreciated.