As Ron mentioned, for brushhogging and blade work you should be using the 'Hold Position' set up on the pump. Assuming you are and that your controls are properly adjusted, the symptoms you're discribing point to a leaking hold position valve, a common problem of the WD line. The good news is, this valve is easily removed and reconditioned. The hold position valve is the assembly sitting on top of the pump that the hoses connect to.
The valve leaks when the check ball does not fully seat which may be due to two different conditions. The first condition arises when the unseating rod physically prevents the ball from fully seating perhaps from wear under the seat (or by someone re-seating the ball too many times as I'll describe next) or gunk building up on the rod. The second is an imperfect seat which can be restored by stricking the ball into the seat firmly with a one pound hammer and a soft punch although you must be sure to remove the unseat rod first and then check for proper clearance afterwards.
Even with a good valve, the hold position might drift up or down especially as you bump accross the pasture brushhogging. Here is where a set of limit chains comes in handy. Set the chains to hold the brushhog at the height you prefer and then only use the hydraulics to lift the hog at need. Drop the hog onto the chains and leave the lift in the down position for normal operation.
For blade work, I'm not sure what you're saying as most of the time the blade is all the way down anyway when working. Are you saying that when you do lift the blade it drops fast enough to see before you can postion the tractor for the next run?