This is a recurring problem. When I vacuum the pool, using a very basic set-up (flexi-hose and vacuum head attached to the pool pole), my pump will vacuum for a bit, then lose pressure, then build it back up, then lose it, etc. I get about 10-15 seconds of good vacuuming, then will lose pressure for 4 or 5 seconds. Then I'll get it back, etc. When it loses pressure, that's accompanied bubbling at the return. Sometimes the fluctuations are enough to detach the hose from the vacuum port at the bottom of the skimmer box. The hose is held there merely by suction (it's not a friction fit, but rather loose).
I always take care to hook the hose up properly (filling it with water and keeping it underwater as I attach it, etc.).
Any ideas on this?