4L80 transmission woe's

I have a 2000 Chevrolet K 2500 Crew cab with a 7.4 Liter engine. 4x4 and 4L80 transmission. The transmission works well in most cases except when it downshifts by itself. It wont shift back into 4th / overdrive and it will shift hard untill the truck is shut off. This is an old body style truck with 80k. If I kick it down into passing gear, It will shift normally back into overdrive and function correctly. It was showing an internal slippage code (18 something or other) and I replaced the shift solinoids and Tq. conv. solinoid. It helped some but it's gone back to acting up. I am thinking about rebuilding the valve body only because the tranny shifts normally otherwise. any other sugestions would be helpful Thanks.

The most likely cause of this condition may be a loss of power to the transmission . This loss of power may be caused by an open in the ignition switch. This condition may also be the result of an incorrect installation of an aftermarket electronic device such as a remote starter or alarm system.


Just a guess!!!