Injector popping on 95 350.

[lowgreen1]

My injectors are making a loud popping noise, not the normal clicking sound but a audible POP. It seems to be just the left one. I have went through two sets of injectors, so I know it's not them. Plus I have switched the injector plugs and it still seems to be the left one. It runs great from idle, it's just at idle it's popping and making the motor stumble. It gets worse as the motor gets hotter. I can be setting at a stop light after driving a little ways and the truck starts to stumble (and you can hear the popping noise) almost to the point I think it's going to die. The mysterious parts is it doesn't do it all the time??? For a little info on the motor, it's a brand new GM crate motor. Almost every sensor has been changed for a new one except the MAP, and knock sensor, and coolant gauge sensor. Timing is set on 0'. Fuel pressure was checked and running around 12-13lbs. I have checked for leaks and couldn't find none. And the TBI has been took apart and cleaned and installed a TBI rebuild kit on it. I'm at a lose, anybody have any ideas?? Thanks