This thread seems to have been hijacked. I gave up after about an hour and skipped to the end. Did anyone come up with an answer to the original question?

I have had the same thing with one light after replacing all my filament bulbs.

My brother found something similar which turned out to be caused by his remote control door openers (I think). Presumably these bulbs can be 'triggered' by any suitable stray radiation (like neons under power lines). Tho the regularity is a puzzle - maybe the leakage effect is the answer.