Fox, when you say your R won't pull 6th, where exactly are you riding, on the flat, cruising the dunes, or shooting comp hill? It should easily pull 6th on the flat, but probably not on the hill. But, if you're running too much tire for your motor, that could be some or all of the problem.
Current Ride (Purchased new in Oct. 84) - 1985 Honda ATC 250R w/ ESR 310cc big bore kit
Wife's Ride - 2009 Polaris RZR 800
Kid's Rides - 2009 Yamaha Raptor 700, 2008 Yamaha Raptor 250
Former Rides - 1975 Honda XR 75, 1979 Yamaha IT 175, 1978 Honda ATC 90, 1980 Honda ATC 110
got sand?