1995 GMC Jimmy engine shakes and stalls

Engine Performance problem
1995 GMC Jimmy 6 cyl Four Wheel Drive Automatic

After 5 minutes of the car being on the car starts to shake and stalls.
the following parts were already changed.
1- spark plugs and cables
2- cap and rotor
3- fuel filter and
4- spider ingector.

there is spark in all 6 cylinder, the ignition system is working but when I checked fuel pressure it was at
50 lbs but the pressure was shaking and going down to 25 lbs within 5 minutes then the car shuts down.

can it be the fuel pump also needs to be changed?

also is the spider injector syncronize or do they all fire at the same time?