Chevy 1 ton auto tranny problem

Anyone have any experience with the 1 ton Chevy tranny dropping out of 4th
gear and back into 3rd while just cruising up the highway ? It does it with
just a constant pedal and suddenly drops down and then almost immediately
shifts back up. The TPS was replaced for this problem but it still remains.
It's just intermittent too. Not a solid failure. The tranny fluid looks
like a yellow instead of red as well. Not sure about that. Otherwise it has
140k on it and runs fine.

Steve m..

No experience specifically, but an educated guess....

I would suspect that a clutch piston seal is extremely brittle and is
not fully sealing (not letting fluid pressure to fully apply) its
respective clutch pack in 4th gear. 140,000 miles is certainly high
enough for a problem like that to arise. If this is the case, I would
recommend an overhaul before something worse happens due to the extra
heat and friction from that slipping component. I don't know what kind
of diagnostics your ECM is capable of as I believe anything over 8500
lbs GVW (1 ton Chevy?) is exempt from OBD-II rules. If your ECM can
set slippage codes, that would be something to check for as added
confirmation.

Toyota MDT in MO