Have you checked to see if your tractor has the proper amount of hydraulic fluid in it? Also, the main hydraulic filter could be starting to plug causing the hydraulic pump to starve of hydraulic fluid. My 4940C did the same thing what you said your tractor is doing. Mine lost all hydraulic power slowly. No steering or the rear three point lift would not lift up or it would jump also.
I changed out the main hydraulic filter and then my tractors hydraulic system was back to normal. This all started around 300 hrs for my tractor. The factory hydraulic filter is a very small micron filter. It takes very little particles to plug it. I would change the filter and see if this will fix your problem before I would go see the dealer. There is a good chance the dealer would not cover this filter change under warranty if it was the problem.
There is a adjustment. The tractor has hydraulic system protection relief valve that is adjustable. If its out of adjustment, the relief valve could be relieving to early causing your tractors hydraulic system to be weak. I would think that this is not your problem, but it could be. If changing the main hydraulic filter doesn't fix your tractors problem, then I would look at the relief valve.