I have the same problem with my Hayward Ultra Vac.

My manual states teh cause as one of the following
-Lack of flow through cleaner.
-Partial blockage of throat area
-Worn Shoes
-Worn wings.

Never been able to find exactly what my problem is, but each time I check the hoses and undo the the captive screw at the bottom to clean the throat area. When I put it back in it seems to work fine...for awhile.

I do know that there is a knob on mine labelled I, II, III on the back