Shifting issues

[Matthew. Collins]

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 

[davester]

I would using a more advanced code reader/live data scanner, that can show you what the transmission is being commanded to do (what gear it should be in, and whether the TC should be locked or not), and that can read codes from the transmission computer (TCM).  Read codes from the TCM to see what it says, and see what gear the tcm commands vs what you feel it's going into.

[Formulabruce]

To "guess" ; I would say the wiring harness inside the transmission is brittle, and or,  a upshift solenoid is not working.

Have seen plenty of these rebuilt and the harness not replaced.