I am sorry I am late in arriving to this conversation, but I just found this forum via search for TCC's and TCM, after I posted this similar problem in another forum.
I am also having this same exact problem, with the exception of the pickup trying to downshift at times, and my pickup going 100's of miles without cycling in and out of lockup.
I am having issues with my transmission going in and out of lockup. I was cruising at about 60MPH about 3 months ago, with no load, without the cruise control on and I felt the accelerator pedal get lighter, (like a spring broke - I did check and I cannot tell if a spring broke as I cannot see very well around the linkage) and at the same time the transmission came out of lockup and then went back into lockup a few seconds later. Over the period of about 50 more miles it got worse really fast, now at speeds over 60 MPH it will slip in and out of lockup about every 5 - 10 seconds without fail. I have had problems with shifting when I first bought the pickup about 6 yrs ago where one of the seals was slipping to the point that it would not go into third or reverse and would take high RPM's to get it to go into second. I fixed this with a Lucas re-conditioner additive that a friend had told me about, and this IMMEDIATELY fixed the problem and the transmission has worked like new since then, until this problem. A shop was the one that told me the seals were worn and would cost more than $4k to rebuild.
This is on a 1995 2500 HD. I have changed the fluid several times since I bought the pickup and it is still red and does not smell burnt each time it is changed.
I am sure it is unlocking rather than slipping, is it is not a slipping feeling, it clearly unlocks, the RPM's immediately raise about 200, and then the trans locks back up and the RPM's quickly go back down about 200 RPM. It also only happens once you get to about 60 MPH or higher, no sooner, I have verified this several times.