Today while finishing up my last yard my engine started to act funny when I engaged the blades. Once I engaged the blades the engine started to go up and down in sound pitch and I felt as if I was losing some power. I checked the air filter, oil level to see if something wasn't right, but everything was fine. It has done this to me once before about 2 weeks ago. This engine has under 100hrs on it and has maintenance done on it regularly. The PTO clutch is bad on the mower so I don't know if that might be the problem or part of the problem. Any ideas what could be causing this?