ok,, you're gonna have to least fill in the blanks a little,,

1.  Had you been working it hard?

   a. was the oil really warm/hot?

2. Which gears does it slip in ?  or

3. Slips just when you let out the clutch to start moving?

4. How often have you changed the hydraulic oil filters?

The "clutch" is no clutch at all, it's just an oil seal. Push foot pedal down, oil seal opens up and lets oil blow by. All the "clutch" is for is "inching" the tractor around like when hooking up to equipment. That aside powershifts are meant to be just shifted without the *clutch* thou granted it's now smooth n' easy like driving a new car down the road when the tractor shifts gears.

The spring which is hooked to the foot *clutch* pedal and goes to the lever under the cab, How old is that spring? Same one that came with the tractor when new?  If I recall the pressures correctly,, it takes something around 800 pounds to hold that oil seal closed. When you get the new spring hire 2 men and a gorilla to swap out those springs. It was all I could muster to get the springs changed by myself. Old one's easy,, new one isn't.

 If you haven't changed the oil and filters that would be a good idea, and look to see if the pump screen in the bottom is plugged.  

Should it "freewheel" when going down hill. ?  No, it should not.