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I tried the 10K resistors across OT1 and OT2 and also connecting mains earth to the TC4 but this didn't help. In fact it was worse with the ground connected.

I then noticed that lifting the cables from OT1 and OT2 up out of the project box made the problem go away. So I hot glued some of the the cables into position and it's working fine at the moment! But I'm still don't really have any idea what was happening.

I'll keep you posted if it plays up again. I still haven't had any more freezes or resets yet either.