That sounds like you have a substantial air pocket trapped somewhere, likely in a rad.
With higher water pressure from faster speeds on the pump, it compresses considerably, increasing the overall loop volume, and the res level drops.
Slow the pump down, the pressure decreases, the air pocket expands back to normal, res level goes up.
Get the air out, and it'll stay pretty much at a constant level, varying only from tubing bulge if you have that much pump and thin tubing.
You may also see a tiny improvement in temps.