5.7L stumbles on accelleration when outside temprature is below approx 70 degrees, every known sensor replaced, Chevy Dealership did a complete check....problem is not always evident. Starts up right away, cold and idles fine, engine revs up with no hesitation, except for once in a while when it's cold outside. I'm having to feather the accellator just short of the stumble and if I just about floor it, the engine wants to stall until I back off the throttle to regain fuel. Mechanics drained tank, checked pump (a-ok) and fuel pressure was about 60%. Could it be the Fuel Pressure Regulator? Thanks...this might be my last repair option, I have spent over $1,500 chasing this problem for a year. Summer time it was running absolutely great!