My guess is that its a pickup issue in the tank..


Few years back i was running low 11's with stock tank, mech pump and a 3/8 pickup..

I always left the tank full...one time i raced the car with 1/2 tank of gas in it and ran into same symptoms you are dealing with.(fuel sloshing around uncovered the pickup)

As others have stated, a cell and electric pump would solve your problems, but putting one of those sump/pickups in the bottom of your stock tank might well solve it as well....