My Chevy sometimes starts immediately and other times th the key has to be turned to start and then off and then to start again before the truck will start.

No doesn't sound like a fuel issue. This sounds like an electrical problem. The first time you turn the key and it don't start, does it turn over with the starter?

Try this turn the key to the on position for about ten seconds and then turn the key to start the truck, do this three times and if the truck starts this way then I would say your fuel pump is weak. Check the fuel pressure when hot and also when its cold...