Fix pH problem now or respect 12/12 lighting cycle

My pH meter went to hell last night. It gave me some very inconscistant readings and then when I tried to recalibrate it, the readings started at 7.3, gradually climbed to 16 (in 7.0 buffer solution, no less) and then quit working altogether.

It seems likely that my readings have been off for a while before this happened, and as a result my reservior pH may be seriously messed up. I kept getting readings in the 7.1-7.3 range so I've added quite a bit of pH down in the last two days to "correct" it. I now suspect that my actual pH could be as low as 3.5, so I need to fix it fast.

The problem is that I switched my plants to 12/12 lighting two days ago, and they're in the dark cycle during the day. So I either have to interrupt that cycle to check the pH as soon as I get back with a new tester or I need to wait until after 7:30 tonight to fix things. If the pH is ok, the interruption will only last 5 min or so, but if it's bad then I'm going to need to drain the reservoir and refill it and that could take 45-60 min.

My gut instinct is telling me that the stress from having the pH badly out of whack is worse than the light stress will be, so I'm leaning towards fixing things now. But I'd like some other input.