Okay, here's the result:
I had a chat with the service advisor (also former mechanic) and he said they hooked it up to the OBDII which came back clean (no stored codes, everything is fine). He didn't really say much about it except it's all normal but no explanation on why (kind of disappointing when you ask service advisers more technical questions and they look at you as if you're speaking in a different language).
Anyway, I did some more testing of my own and discovered that the power surge interruption only occurs when the A/C is on. I tested it twice on the same straight, level stretch of road (at 25 degrees C); when the A/C was off, the power surge was linear and uninterrupted. As soon as the A/C was back on, the power interruption occurs around 5,250rpm.
So now we know the culprit for the power surge interruption, it's the A/C.
Happy motoring folks
