I hope you are aware, if these are HD heated grips, you can not buy, just the grip! If yours are like mine, the left grip contains the "thermostat" and the heater control. This comes as an assembly and is the bulk of the price of the complete grip set!
That, however does not solve your problem. Clean the grip with isopropol alcohol. It sounds like something has gotten on the grip and "attacked" the rubber!