i have the same problem, it seems like the skimmer with create not alot of bubbles and then all of the sudden it will just kick in and work again and create lots of bubbles and then it over flows, my skimmer water level is always constant and it happens when im away from work. so no feeding is done. i had 5 days of great skimmate and just when i was going to empty it after work bam, all back into the tank
this is the second time it happens to me, only had it for 2 weeks so maybe break in period. but i have no idea either. i have no idea what the reason for it is though. nothing changes in my tank from when it works and then it overflows.