1991 Chevy Truck chevy 1991 pickup truck , will start but s

check fuel pressure and ignition switch.
Let us know
Put a meter on coil B+ and see if voltage goes away when you release key to run position

we''ve tried everything, someone told us it might be the distributor but it wasn't, they think the timming may be messed up but not sure we are going to find out today, um I need a list of everything that could be it, thats how were doing this now, a shot in the dark kind of thing, we've replace just about everything, but we may have missed something :(

we've checked the ignition switch also checked fuel pressure neither were the problem, and we've replaced the coil , but still does the same thing, but after a long amount of time cranking it up and it shutting off it will run longer and longer until it seems to be running fine then it shuts off but after every time we crank it up it run;s longer and longer

can someone please help