I missed the intermittent part. That points to what Jan and Mark mentioned and not to an air leak. If the water is low and a vortex is forming in the skimmer and you don't want to add water then put a few small rocks or something in the skimmer basket to break up the flow pattern and you should be OK until the level gets higher. I don't have a weir 'door' but an older style floating circular one that sits in the basket. That has a tendency to stick at high water levels and do the same thing because the flow over the top and into the basket isn't enough to maintain full flow and I would get the same intermittent pumping action.
Al