Dunno, but he wasn't able to replicate it even on the crap back roads around Benson / Wallingford or on the ramps so I said might as well replace the passenger side as the drivers side was done last year and gives me peace of mind knowing it won't need doing. He replaced the temp sensor and said it still isn't working so probably something else (wiring at the back of the clocks like I've seen A4/A6's suffer?), but it worked this morning all the way in when I picked up so I'll keep an eye on it. He also found a leak in the radiator which needs doing at some point soon.
It also needs a service, it had 1500 miles left before the service was due then change to Service Now suddenly about a week ago.
It's a good thing I've got a car savings account for maintenance now so I can get all these things done without seeing it come out my normal account