I was towing a tandem trailer with 2 cars on it down to indiana from michigan (6 hours) i had the truck at 70-75 the whole way with no problems. the stock trans held up which i was surprised but at 60-70mph the trans would unlock and slip like crazy but once it locked at 70 it would stay locked. at 70 mph im at 2000ish rpms. once at 55 i couldnt the torque converter to lock at all!!! and i had to hook up my lock up switch on the side of the road What could cause that? I left the switch in when i went to the dunes pulling a trailer that was a bit lighter but still the same problems at 60-70 even with the switch in. i was on EJWA at level 5 the whole time and even when i put it down to stock it would do the same thing at 55. Is my torque converter done? are my clutches doing all of the work? i also hit 1200 egt's going up a hill with the trailer. What temperature is bad for the turbo? anything i could do to avoid this?

Thannks!
-nick

 

Sounds to me like your TC is on its way out.