92 with a low power/idle issue

[Develdog1106]

I have a 92/auto that runs perfect when cold. Once the truck gets to operating temp, the idle drops way down when stopped and in gear- it feels like the torque converter is trying to lock up (I know that's not what the problem is, just what it feels like). When I take off from a stop, it feels like the truck is dragging a battleship anchor for a couple seconds, belching a lot of black smoke, and then takes off like a scalded cat. When stopped and I put it in neutral, it idles perfect. There are several threads with similar, but not identical, problems; this is what I've done so far:

-Transmission filter and flush (done every year with full synthetic)
-TPS checked, good
-Computer swapped with a known good spare one I have
-External 12v applied to KSB (and tried removing the wire also)
-Lift pump replaced (from Cummins)
-Injectors bled (twice)

Everything I've done so far has made absolutely no difference. I did stop by a local diesel performance shop and explained the problem to him. He had a quick look at the truck and assured me it wasn't a pump problem.

Just got the truck back from the local Cummins Dealer, who put two hours of diagnostics ($220) in it and found nothing wrong. I declined their request for two more hours. They checked air filter restriction, leaks, air in the system, fuel contamination, transfer pump, and overflow valve. All checked good. Their next step was to adjust valves and pump timing.

Anyone have a direction to send me? I would seriously appreciate any help- this is getting extremely frustrating!