JD Hydraulic pump cavitating

HeyIve got an old 3350 (i think it is) having trouble with the hyds, at low revs i have no hyds, rev it up no worries, drop it back to idle and i have pressure for 10-20 seconds, hyds then start to shudder like its low on oil (its not) and stop completely. rev it up they shudder for a bit then work fine, idle down, shuddder etc. etc etc...
any ideas or common problems with these things????
Cheers.

Your problem will be on the suction side of your hydraulic system. My guess is you've got a leak in your system somewhere between your pump and your reservoir. The leak is small enough your pump can still pull suction when you've got the RPMs raised, but when you lower RPMs and lower the suction, the leak is letting in enough air to overcome the suction. Then your pump loses fluid and you have no hydraulics. You still have pressure for a few moments because there is still some fluid in the pump and in your system back to the leak.