Is it new? If so, mine took a few weeks to break in and be consistent. I had to adjust up and then down, then up a bit, then down a bit. Eventually it broke in and I found that magic point where is worked 24/7 without varying (unless I have a water level issue - see next paragraph).
The other potential issue is the water level. I have never used a HOB skimmer, but if your water level in not consistent, it will most likely impact your skimmer. If you have an ATO and you adjusted the skimmer when the water was at a lower level, when the ATO filled it back up (or you manually filled it back up) it could cause your skimmer to overflow. If I overfill my sump past my magic mark, it will overflow (fortunately just overflows back into the sump).
Of course it could also be as Larson pointed out, recent changes to the tank (heaving feeding, water change, hands in water, etc.).
If it is new, keep making minor adjustments and you will find the right mark. If you have had it a while, I am betting it is a water level issue.
Those are the most likely senerios. Let us know how it works out....