I have a '94 E320 wagon that has suddenly started downshifting on its own. It usually starts happening after the car is warm, maybe 10 - 15 minutes into driving. I'll be cruising along at 65 and suddenly it shifts down from 4th to 3rd and the RPM's jump from below 3k to between 3k and 4k. It has also on occasion downshifted to 2cnd after downshifting into 3rd.

If I shift into neutral then back into drive it will go into the correct gear, but then downshift again after a few seconds.

I've checked the fluid and it is clean and at the correct level.

I disconnected the kick-down switch under the gas pedal without any effect on this problem.

The shifter bushings are good, no slop in the shift selector.

This morning I disconnected the Bowden cable thinking that perhaps it was out of adjustment. No change in downshift problem.

Additionally, this car has an updated (non deteriorating) wiring harness and throttle body. The engine idles and runs smoothly.

Can anyone advise what I should try next?