I recently bought a dual speed pump, 1 HP for a 18' round above ground...to work with a Hayward Perplex DE filter

My problem is....

Originally when I bought it the high speed worked fantastic and as quite honestly impressive. I vacuumed with it...perfection.

I started er' up last night to vacuum and low puts out a trickle (out of the return) and high works about as well as low used to. Also, air bubbles spit out of the return...AND the pumped works slow for 20-30 seconds spitting out air bubbles and then it's like a toilet bowl flush where everything rushes through and the return works/skimmer works completely normal. It works normal for maybe 5 seconds and back to low return water and low suckage.

Any ideas?!?!