Help, Please: 87 300SDL Trans Problem

[pstutcher]

Please help, I need the collective expertise of the members of this forum.

I am presently stumped by a seemingly very unusual transmission problem that neither my indie nor myself can quite figure out. Any help would be appreciated.

I have recently purchased a very nice 87 300SDL with 240K miles as a stablemate to me 92 300D with 380K. Driving it home from New Jersey to Michigan I noticed a slight problem at the start of the trip that seemed to resolve itself after a while: Approaching highway speeds, the transmission was very reluctant to come out of passing (3rd?) gear. Eventually it did and I drove it home and for several weeks with no problem.

I took it to my mechanic for a laundry list of repair and maintenance items and mentioned the shifting problem to him - he subsequently changed the very dark tans fluid and filter and suggested that the problem seemed to be alleviated - and it was, for a couple of more weeks.

Recently, about 8 miles into a freeway trip cruising at 75 on cruise, the trans dropped to passing gear and was coaxed out by tapping accelerator to hit the kickdown switch. This worked more and more reluctantly for 1-2 more miles until it stayed in the passing gear at ~4000K on the tach. A few moments later, the trans decoupled from the engine completely and I coasted to a stop, the engine able to uselessly rev. the whole time. After shifting to park and back to drive, the car was able to go again, but would not shift past 2nd gear.

When the car cools off, the problem goes away and starts over again on the same cycle. It seems to be related to the trans running for 6-8 minutes at freeway speed (60-70 mph) and goes away after it cools down.

My indie is checking out a possible stuck kickdown switch, though that wouldn't explain the repeated delay in failure. He has checked thre vacuum lines that control the trans and they appear to be operating normally and the trans fluid pressure also seems to be OK.

Has anybody experienced anything like this? If so, how was it corrected? If not, what would you suggest? I don't want to replace this trans in order to get my car back and running, especially if it won't correct the problem.

I understand it to be a 722.321 transmission.

Any help would be profoundly appreciated.

pete