XC70 wagon 2004 ECU or transmission problem?

[fjccpi]

Good day all. First time posting

I have a 2004 XC70 wagon, 80,000 miles
It has developed an intermittent electrical problem I believe.
The car will suddenly ‘buck' for a second as if it stalled or is trying to downshift. The inside lights will go on, and a ‘left door open' message will appear. Sometimes the dashboard lights and indicators will go out as well.
The transmission was flushed by the dealer for the first time at about 70,000 miles. It was dirty at that point according to them. 75,000 mile service was performed, and the software upgrades were done.This was about 6 months ago
The problem continued after the service, once the weather got hotter (in New Jersey) One volvo dealer had no idea, another said it was a $1,200+ problem:
‘Fault trace to internal fault in DIM P/N 8602763, 8633289'
From browsing I have picked up the idea it is the ECU. From other posts, it sounds like I should have the transmission flushed and check for software upgrades. I am not planning to attack this myself. I'm looking for clues to discuss with a mechanic.
Any advice?