The issue concerns the torque converter lock-up logic. If the problem can be easily duplicated (i.e. it does it all the time), the diagnosis is pretty simple. Unfortunately, you'll need an SDS diagnostic computer. The torque converter lock-up feature can be temporarily turned off using SDS. If the problem goes away then the TC is the cause. The new ECU has modified software which affects the TC lock-up logic.

Lots of indies have SDS, so you may want to check around and see if you can find someone locally who can help you. I don't know if any of the after-market machines are capable of transmission diagnosis on MB's, but it's certainly worth checking out.

Whatever you decide, make sure you diagnose the problem before spending $$$$$!!! Best of luck.