I posted in this forum 2 years ago about a problem I had with replacing three fuel pumps in three years. Well I guess it is better late than never to share the answer and this is what I found which BTW many left scratching their heads about this issue.
When you have problems with your electrical on these trucks - check the small single wire which leads from the battery along the frame rail up to the power junction box on the firewall. The electrical junction box that has the plastic cover over it. WHAT I FOUND IS THAT SINGLE SMALL WIRE OVER THE MANY YEARS OF OPERATION HAD RUBBED ON THE FRAME-RAIL NEXT THE THE BIG STARTER WIRE CAUSING AN INTERMITTENT SHORT. THIS IN TURN WAS KILLING THE FUEL PUMPS OPERATION INTERMITTENTLY. So check that wire FIRST because it supplies power to many other functions on the truck as well. Success finally! I Hope this can help others who might be having similar electrical problems. My 88 GMC is still going strong with over 247K on the ticker. Long live GMC - Hopefully .....