MF 1240 4x4

[smittyd]

I have a 2000 1240 600 hours ,used mainly to plant and maintain foodplots and some mowing of roads and areas around camp .When I place the tractor in PTO and start to mow or till after a few minutes the PTO pops out of engagement and I have to put the lever back into PTO to engage it again and after a while it does it again.I have owned since new and have maintained it regularly. Any ideals on this proplem? While in PTO there is a mild clicking noise coming off the top of the assembley under the seat .....