1989 GMC Sierra Engine idles but stall when put in gear whe

Hi. I''ve put in new spark plugs, wires & ignition coil and still have the identical problem. The truck starts & runs fine when cool. When it heats up after about 40 minutes of idling or 20 minutes of driving, it stall if I stop at a stop sign, or idles rough in neutral - then stalls when I put it in gear, unless I rev the RPMs up to about 2000 before shifting into gear. Once it gets really hot (not overheated !), if won't even start until it cools down. Any ideas. I've thought of the fuel pump, but as I understand it (unless I'm wrong), the pump is in the gas tank, so I don't see how it could be a heating issue ???