It doesn't happen till the trans is good and warm (150*-170*). When I put it in reverse, the truck lugs way down like its under heavy load. When I pop it back in neutral, it idles back up.
Trans is a local rebuild with about 10k miles on internals. TC is a PI Stallion with probably 20k on it IIRC. Everything pulls good and shifts are crisp. When the trans temps are under 150* or so, I dont have this problem while in reverse.
Matt R. (who I bought the trans and converter from), said it might be a leaky Torque Converter Clutch Solenoid in the Valve Body.