Is my transmission slipping? I can't tell for sure.

[rsualk]

I finally got my recently acquired 1987 300D to make a full 13psi of boost. It had a clogged CAT and could hardly make more than 5psi with lots of smoke (and 40psi of backpressure). Now I get lots of "go" and no smoke, but I also get 3000-3200 rpm in 4th gear from about 45 to 65 MPH under acceleration. It never revs past 3200 below 65 (in 4th gear), but 3000 rpm at 45 in 4th gear while accelerating just below downshift level ? That seems wrong. The car basically accelerates smoothly and strongly from 45-65 all while the engine only increases by 200 rpm - almost like it's got a hydrostatic drive!

I verified the trans vacuum and adjusted the modulator to give crisp shifts for 1-2 and 2-3, though 3-4 is a bit soft (but *never* flares). I also changed the transmission fluid, but it made no difference. I thought the old fluid smelled very slightly burned, but I could be fooling myself. In any case it wasn't obvious.

So I'm trying to figure out if the torque converter on this model is designed to slip a lot or if my 4th gear clutch is slipping and somehow hasn't burned up yet.

When stopped, if I mash the gas and the brakes at the same time, I get about 2500 rpm and then usually one of the rear tires breaks loose (maybe a stuck caliper - I haven't checked yet). In contrast, my '83 300CD, which has the occasional 3-4 flare, never quite manages even 2000 rpm when not moving (but it runs very well too since I installed Monark Injectors, newTurbocharger bearings and seals, adjusted wastegate to 11.5psi, etc...).

Any thoughts? I'm especially interested in the behavior of other 1987 (OM603 based) 300Ds.