i bought a 98 12 valve today and it will die at wot and when i let off on it if i get on it very hard, it will also die if i hold the brake in drive and tap the pedal, it idles fine and all i just can fig it out

 

i would start by changing the fuel filter, the dieing at idle when you tap the pedal is usaully a sign of low idle maybe only a 100 or so rpm. but i would check the return pressure regulator on the return line comming off the pump i have seen that cause the stall at wot and if you let off suddenly. if none of that then i would check the set up on those 4000 gsk's.

Chris