1989 Buick Century car cutts off

Hi, I have a 1989 buick century custom with 3.3L engine. Now, my problem is all about the car's shutting offs after it's been driven for about 20 minutes. I already consulted some techs and so I changed everything they suspect that's causing the problem. Fuel pump is new, tps is new, AIC valve is new, ignition switch is new, and also I have changed the ignition coil, and lastly, I tried changing the ignition module with a used one but there is no change. The car still stalls. I just don't know if the ignition module that I got is defective. I there something that I missed that's causing the engine to shut off?