Rapid pH rise, due to temperature?

Well, pH will increase as temperature decreases, but that seems like a big jump. Your feed hasn't become more alkaline though (if it's caused by temps), the neutral point on the scale moves along with temperature as well. That is to say 8 might be the new 7. More likely scenario is you must recalibrate your meter. Or when you originally measured it, your reservoir wasn't thoroughly mixed enough. I'd get it down to about 6.0 or so and keep monitoring.