[Polo on]

Hi John, you seem really helpful and knowledgeable, so I've aimed this at you, or anyone else with this skill level.
So yesterday morning I tackled the ETM replacing the main moving part with an electronic Sacer unit (I am a biomedical electronics engineer, with a pretty high level of mechanical skill from working on IV pumps and similar). I used the basic instructions that I found online, and did not have the information about voltage testing, so I might tear it all open again to take measurements and possibly re-adjust the magnet.
Everything came apart nicely and went together well, and a short trip around the block was enough to convince me that I could re-pot the connections and seal it up fully, which I did.
However, at the end of a 30 minute round trip yesterday evening, the car was worse than I have ever experienced. Lots of surging whilst sat at traffic lights, making me sound like a boy-racer, then it stalled on me twice, and went into limp home mode for a while. I am thinking I have not found the true cause of the problems.

Finding more information online with voltage measurements last night, after the job was all bolted up was annoying - but if the magnet position was badly out, surely the faults would exhibit all the time and not intermittently?
I ran Vida again today and the error codes I have this morning (having cleared everything after the job) are:

CEM-1A5D Communication with ETM control module signal missing
CEM-DF13 CAN-H, High speed network
CEM-DF16 CAN-L, high speed network
ECM-902A Communication with throttle unit- Faulty Signal

Now I'm scared to take it out at busy times or for longer trips until I can remedy this.
Has anyone any ideas or suggestions? Seen this before maybe?
I replaced the MAFS in December, as that was the cheapest item in the chain, but that made no difference.
And in the cold the car is idling perfectly, and throwing no fault codes.

Paul