Hi,
I recently had the same problem on my 90 240, although the stumble went away with a fully warmed up engine.

After swapping a known good ECU and AMM and checking fuel pressure without effect
, I went on a fairly long drive and noticed my gas mileage was down.

That led me to think that a small leak at the header pipe to cat connection that I had been negligent to fix might be at play, and indeed replacing the gasket cured the fault and improved the mileage.

My guess is that the O2 sensor was sensing a high oxygen situation and the 951 ECU was sending more gas to compensate. On a cold engine this might have caused a momentary flooding and stumble on acceleration.

I recommend that you check for any leaks in the exhaust between the engine and O2 sensor.

Good luck, Peter