poor circulation (cycling up and down)

[crperkins]

I have an inground pool (about 18,000 gal) with a stayright pump powered by and emerson 1hp motor. Regardless of what mode i'm operating (normal circulation or manual vacuum to waste), i can't get full circulation. as soon as it cycles up to full circulation-it immediately drops in pressure (and water circulation).
I've replaced the pump seals (impeller ok and cleaned), basket top and o ring, and cleaned the D.E. filter (LOTS of d.e. had accumulated).
Any ideas?

Thanks!