Have you seen this before? Please let me know as I've stumped the dealer, a good mechanic and a good transmission shop...

Symptom: At a steady, non-accelerating 25 MPH (~1500-1800 RPM) in D or D2 with or without overdrive, with or without 4H engaged the vehicle experiences a lurch/clunk. The RPM shows a ~4-500 drop each time it happens. This behavior can be seen at ~45 MPH because it falls at the same RPM range.

At first I thought it was a transmission problem so brought it to my mechanic who after hearing the symptom thought taking it to a transmission place would be wise. The transmission guys took it for a test drive and recommended rebuilding the existing tranny. After completing the rebuild I picked up the vehicle and the symptoms remained. The check engine light came on with a p1750 code.

Took it back to the transmission shop and they replaced the valve body- (which they had already replaced once as part of the rebuild). However the symptoms remained.

After driving for ~3 month, with the symptoms the p1750 engine code came back. Took it back to the transmission guys who added an additional grounding strap to the engine and transmission and replaced the throttle position switch 'on a hunch'. This interestingly enough made the problem worse!

Took the vehicle then to the dealer to see if any additional info or diagnostics could be found. They found an Mistu error code 49 which points to the torque converter or the valve body. They recommended rebuilding the transmission again!

Any thoughts? Is this really an ECM problem? I believe the transmission shop really did rebuild the transmission and really did replace the valve body twice so it seems unlikely that it could be those again!