Please Help!!! no power! 05 brute force 650
 

i have a 05 brute 650 with 90 hours i was rideing one day and i had all the power in the world 63-64 mph until my way back home and the power started to jump from 60 to 50's back to 60's i felt it happening but needed to get home. until now i can only go no faster then 40mph and takes awhile to get there. everything is stock. i changed the air filter, reset the belt and ran some seafoam through the tank which i do every other Tank anyway but nothing i still have no power and cant even spin the tires. idles and starts right up just fine with no clicks or anything please help! thank you!