I have a 7,000lb travel trailer. While towing the trailer and when I go to down shift from overdrive into 3rd the tranny starts acting like its going in and out of lockup. When not towing I don't have this issue.
I do think it takes a little long for lockup to engage.
Should I replace the solenoid (lockup) or should I go and direct wire/switch the lockup?? or is there another issue I'm not seeing.
I just rebuilt the tranny last fall with $1200 in parts and its been running great.

 

Chances are your alternator is getting a little worn and causing electronic noise and its interfereing with the transmission. When I had my 2000, I aluminum wrapped the brown with orange tracer wire from the alternator to the battery. I also put on 2 good sized ferrite chokes also. That solved my problem.

 

I had an 01 with a 318, and my uncle has a 98 Cummins, that the converter would release then lock back up randomly. Turned out there was a bad wire somewhere that dealt with the throttle. My uncle got all his money back when they rebuilt the trans, replaced the valve body, then the trans controller, then the valve body again, and an independent shop told him there's interference, go have them check for a TSB. They spliced in a jumper and fixed it.