GM-GMC: fuel pump, chevy 350 vortec, fuel pumps

Question
I have a 96 1500 chevy 350 vortec when you turn the key on the fuel pump will come on and sometimes it will not come on and want start what could be causing this

Answer
Quite likely a faulty fuel pump.  Using a test light, unplug the harness going to the fuel pump and check for power and ground, if my memory serves me correctly it will be the black wire for ground and the grey wire for power.  Cycle the key five or six times and make sure you have power at all times.  You may also try giving the fuel tank a bonk or a light kick one of the times you don't hear the pump run.  If it starts running, you can be sure it is a fuel pump.  From my past experience fuel pumps are not uncommon to malfunction on those trucks and would be surprised if it was something else.  A few quick checks will ensure you don't replace something  you don't need to.
Hope this  helps