I was thinking along the same lines as ReefPilot in that ambient air pressure may be one contributing factor. If you think about how a liquid barometer works, our skimmers are not that different, a large tube filled with a column of water. Any ambient air pressure change, will have a small effect on water level. Perhaps more so in some skimmers than others. I've noticed that whenever I put my hand or arm in the display for whatever reason, the skimmer quickly produces less useful foam, so the little bit of oil on my skin is all it takes I guess. I therefore assume that the opposite could also be true if even a small amount of whatever causes more foam somehow is introduced into the tank. Not sure if pH would affect skimmer performance, but might be worth researching since many of us have pH swings between lights on & lights out.
Funny, I've read a lot over the years how finicky & maintenance intensive a counter current, wooden air diffuser skimmer can be to dial in, but I rarely have to touch the valve controlling water column height on mine. The only time it has ever overflowed is when I've inadvertently closed that valve a bit too much, keeping the water column too high. Starting to really appreciate some of the pros of this older technology since I do little to no maintenance on it, (DIY air diffuser every 3-4 months). It's quiet & between the small powerhead moving water & the air pump driving the air diffuser, it uses very little power. If anything, it's more likely to underflow than overflow. This is definitely a pro in my case, since it's a hang on display skimmer, & any major overflow would end up all over the floor. I have a drain line to a 2L bottle & an empty salt bucket sitting underneath the skimmer for a little added insurance.
These are older photos of my skimmer. I now run a larger air pump to drive the king size DIY air diffuser that I change out every few months.
