Idk man, but I have the exact same problem. QDM codes that come on right with tcc lockup, and turn off at 34mph. Oddest thing ever. My fans don't turn on in traffic, but shorting the A and B terminals to check codes, turns them both on. Also my EGRs don't appear to be working, and when they do work my idle seems fine, IMO. I do have a few vacuum leaks, you don't by chance have a messed up PCV system, the hose from the valve cover to the TB, and the valve itself and gromet on the intake?
I too would like to know the culprit here, for the most part it got a lot better with new intake gaskets, but it'* still there, and its only every 2 out of 5 times i have the car out of gear does it idle around 2k, sometimes it'd be 3k as well... but that has long past with new intake gaskets.
'89 SSE - GX3 -190k, 155k on body - Back for another ride.