HELP!

My 95 sat sl1 has been acting up since the thermostat stuck closed, replaced thermo and coolant temp. Sensor. Coolant system then check out ok. Drove five miles to check running condition and fuel up. Tried to start after fueling, would turn over, but not fire the first three tries like it wasn't getting fuel. Since then ( three weeks ago ) I have been driving it only to and from work. The car runs great, but when idling it will stay at about 1100 RPM for while then jump up to 1400 back down to 800, then steady back out at 1100. It bounces like this a few more times then eventually stalls. After stalling will not fire until I let it set for a few hours. I have replaced the fuel filter, throttle position sensor, and idle air control valve. Checked all fuses/ relays and cleaned the EGR valve. Nothing helped. Could it be the fuel pressure regulator, fuel pump, or fuel level sending unit? Perhaps a vacuum leak? I don't know. Please help with any info or knowledge of the problem. The car is a 95, but the engine is a 99. In 2005, one of pistons broke so engine was replaced. Its been runnin' like a champ 'til now. Thank you for your help.