I have a 2009 Brute Force 750. When I first got it, I was amazed at the power it had. I could hit 65 MPH no problem. Recently, it seems to not have the power it had before and it will only go about 51 MPH at the most. It feels fine up to about 10 MPH then it just feels very lathargic. I was told to change the spark plugs so I did that using the factory NGK plugs. I also checked and cleaned my air filter. Changing the plugs helped slightly but it still seems to not have the power like it did before. Any suggestions?