Seed grower

Help!! sector 1000 under 500 miles. I am second owner. Ran it for few hours then it started to slow down after I got it up to 45 in high. RPM’s dropped and speed dropped with throttle to floor. Stopped and put in neutral and engine runs fine. Put in low and run at 35 not problem. Go back to high and hits 45 and slowly derates. Engine sounds fine. Air filters new, belt looks good