i have the vortec 350 it ran fine until i got it stuck in mud and water last year now it runs and idles smooth until it gets warmed up then backfires and tries to shut off when driving, if i let off the gas when this starts it smooths out but if i give it gas again it still has the problem, if i let it cool then its fine until it warms again. seems like it lacks alot of power for a newly rebuilt motor.