Possibilities:
1) loose bulbs in fixture, try rotating them in their holders
2) dying ballast, replace
3) loose traveller at the switches, check all travellers
3a) loose neutral at fixture, check neutral at fixture
4) loose neutral at panel, check neutral at panel

Most likely cause? Could be anything. Start with the easiest, which are loose wires or connections. If the fixture is more than a few years old with lots of on/off cycles then I'd look at the ballast.

Can I assume, that since there was power in the both switch boxes when the light was off then that would rule out bad breaker? or loose connection in panel? (no volt reading taken, used my little pen tick tester)

Does the ballast have a thermal cut off? Or can be effected in this way by humidity?

Yes, I would assume the breaker is OK. But you can't rule out a loose neutral in the panel. I'm not sure about thermal cutoffs in ballasts. The older style oil filled
ballasts did not have thermal cutoffs AFAIK. The ballasts can intermittently fail when they start to age.