I'm going to go with running hot as she did have to work hard pulling a drag in soft snow with the tracks on, I'll add plugs to my "spare parts on hand" listJust some of the reasons that cause spark plugs to foul:
Overheating or running very hot
Rich fuel mixture
O2 sensor fault
Restricted exhaust/restricted air intake
Excess carbon buildup
Worn piston rings or cylinder walls
Work valve guides
Head gasket leak
Injector leaks
Improper gap
Ignition timing delay
Poor maintenance
Driving habits (low speed, idling)
Incorrect plugs installed
Wrong/bad fuel used