I change the oil every 2500 miles, I have year old plugs, I use fuel injector cleaner every 3 fill ups, and I use Valvoline high mileage oil, plus synthetic blend. I also have a K&N filter which was cleaned a year ago. Jus got my battery recharged a few months ago, along with a new alternator. I don't have anyother highpower electronics sucking juice from the battery when cranking. However, I still have the stock fuel filter in. How often should that filter be replaced? Could that narrow down the problem?