Sounds like their is something partially clogging your drain line.
I found yesterday that one of my drain lines was a little slack in flow, causing my tank to ride a little high too. I never put the screens over my Dursos, because I put covers over the overflow boxes to block light from getting into the boxes, and causing algae, both in the box and in the overflow combs. I didn't screen the Dursos because I wanted maximum flow. Apparently, before I put the covers over the boxes, an Astrea snail crawled in there. I put a long, skinny metal rod through the hole on top of the Dursos, and could feel/hear something in there. Assuming it was a snail shell, I gently pulverized it, and it fell into the sump.....Astrea! Flow was back to normal.
I put those covers on like 8 months ago, so it had been in there a long time, but only disrupted flow here and there. I had noticed less flow out of that box before, but couldn't figure it out. Luckily I found a way to clear it without taking it all apart!
Anyway Sniper, sounds like something might be in the drain pipe.