The voltmeter doesn't realy mean much it's the cranking capacity of the battery that is important, it might show full volts with no load on it but when you try to start but when you put a heavy load (starter motor) it could be dropping too low for the coil unit to giveout a good spark, but by bumping it the coil will have the full power available, Go to a tyre & exhaust place and ask them to do a "drop test" on the battery
nigel
July 2008