OK, you said GMC. Does it have Electronic Spark Control (ESC)? It will have a badge usually on the back of the truck. Anyway, you can tell if it has ESC by the fact it has TWO connectors coming out of the metal distributor housing (not the cap). The ESC is intended to alter the timing under load but sometimes it gets a mind of its own. You can easily remove the ESC from the function.

First thing to do is unhook the spark control computer lead from the pink wire and hook the pink wire directly to the dist. cap. Make sure you plug it into the cap at the BAT terminal. The second thing is to locate the 4 wire connector that feeds into the ignition module. There's a brown (D), green (C), white (B), and black (A) wire there. The letters in () are on the black clip that the wires come out of. You jumper the green (C) and black (A) wires together by adding a short wire stuffed temporarily into the back side of the connector.....just use a short piece of wire and strip each end and stick it into the connector. IF you have a problem, pull the jumper out and you are back to normal setup. IF this jumper works (after testing for a few days), you can permanently make a jumper by cutting the wires and soldering them. You could also replace the 7 pin module with a 4 pin module to elimnate the ESC feature.