843 intermittent Loss of hydraulics

[weave]

I have a 87 843 I bought used recently. I was on a hill with it and lost control of all functions and coasted to the bottom luckily. After a brief pause all functions started working fine again it seems to do this randomly. The machine has always had a whine under steering load similar sound to a cars noisy power steering pump. I have noticed that when it acts up this noise goes quiet. Any one have any advice with this issue? I have checked fluid level and for leaks

 

[Tazza]

Almost sounds like a lack of charge pressure, but i'd have thought the issue would be there all the time, not just on and off.....
Do the hydraulics still work when it does this? As in push the bucket down to stop rolling?