A few more ideas...
The generator pre-2007 only put out 375W, the 2007 twin spark model's generator puts out 400W.
If you are you running any other extra electrical equipment besides the heated grips you might be using more than the generator puts out and as the battery voltage drops the controller switches the grips off.
Do they only switch off when you idle, like at traffic lights, or do they go off even when giving it the beans? At lower revs the generator will be putting out less power and if the battery is old then it won't take long for the voltage to drop too low for the controller.