I have a 2006 chevy 2500 hd 6.0 4x4 that is doing some funny stuff. The trucks tranny was rebuilt 3000 miles ago and had been shifting great until it just started shifting into what seems like overdrive after second gear and then the pedal goes dead. Will not down shift and accelerate until i let off the gas, and then it juat down shifts to 3rd and goes right back into overdrive . Any ideas would be appreciated. It only does it intermittently and may go right away or not happen at all. Thanks in advance