Hi Alasdair,
Given your vent is fairly close to the wing leading edge and also close to the wing root/fuselage junction I wouldn’t be surprised if the turbulent air in this region was leading to pressure variations in the vent. These are just as likely to be negative as positive. Also on a low pass or landing you are likely to be flying with the wing at a higher angle of attack which again could change the turbulence in the vicinity of the vent. Given that you have already proved that pressure variations in the vent affect the ECU’s ability to control the idle Im fairly certain this is what’s happening.

If the tree hadn’t severely modified the aeroplane I’d have suggested adding a U bend to the vent so it was facing backwards. This would be much less likely to see pressure changes even if the effect is to reduce the pressure in the vent.

Hopefully, you will build another and take less time to do it!

Malcolm