You may be losing the ground. Disconnect the electrical connector at the clutch and connect your test light across the two terminals, key on, with the PTO switch engaged (you may need a helper in the seat) and wiggle the harness while watching the test light. I'm not familiar with the clutch on your model - is it completely round or is it triangular shaped with three bolts at the perimeter?
Measure resistance of clutch coil and look up spec for your particular clutch for comparison.
Either losing power/ground, out of adjustment (only on adjustable clutch), or defective new clutch.