batterys are only good for about 2 to 3 years and fail quicker when not in use.
check the voltage is around 14volts at idle if its not the reg maybe faulty.
when my bike had this problem the battery checked fine the first time but the voltage to the servo motor would drop and spike from 12v to 6v i thought it would have to be a short in the wiring.
but it turned out to be inside the battery itself i can put a multimeter on it and it reads 12v then if i shake or tilt the battery it will go from 12 to 9 volts and would be worse under load of cause.
any battery or auto electric place can test it for you this is your best bet as ive seen heaps of batterys that test fine 12volts on a multimeter but fail under load test conditions