Since it sounds like it's the 3PH lowering slowly (based upon your comment about the shredder), It's probably the "Rate of Drop" knob turned in a bit.
This knob is located on the panel directly behind your shins (when you're setting in the operator's seat).
Try adjusting that knob first...
Personally, I prefer the 3PH to drop a bit slow...say 2-3 seconds.
If that doesn't help...the next step is to replace your hydraulic fluid, filter and clean out the suction screen. This process will be in the Operator's Manual that came with your 990.