1988 Riviera Idle stalling

My 1988 buick riviera 3.8L 3800 engine keeps Idleing on acceleration and stalling from time to time. I change the fuel filter, air filter, distributor coil, spark plugs and wires, I also changed mass air sensor but nothing, engine only runs a little smoother but I still get stallling and Idling even sometimes when car is in park. I checked the fule pressure and gas shoots up out the valve. I also can hear fuel pump working when key is turned. And also my seatbelt warning stoped working at the same time. I aslo used fuel injector cleaner and still no change. And there is no check engine light either. Can someone please help me out I will gladly appreciate it.