Hot Pamp

[builtogo]

Sunday during a ride at CATRA (Fishouse, NY), a rocky challenging set of single track trails in the southern Adirondacks, my '04 Pamp 280 began running very hot, blackening and partially melting the plastic behind the exhaust. The engine was low on power and could not be be "cleared out" by holding the throttle open. Cooling fan was operating and coolant level was high. Changing plug, running without air filter or muffler offered no improvement.
I later found mud on the lower two or so inches of the radiator. Could a hot running engine cause the problem?