John Deere 4300 Losing Power

I have a 2000 JD 4300 with 1,100 hous on it. The past few times I mowed, the tractor seems to suddenly lose power. RPM's gradually drop from 3,000 to 2,000 or so as if the motor is under heavy load (but its not under heavy load). This carries on for a few minutes or so, then suddenly back to full power. Runs great for an hour or so, then suddenly lose power again for a few minutes. Engine temp is OK, no black smoke coming from exhaust, no strange smell,... Any ideas on how I can trouble shoot this problem?

Thank you