Andrew...couple of things you might check...when it's running, check the battery to see how many volts are there...S/G could be the culprit...also check the ground connections....could have a bad cell in the battery...if you have a 12v test light, you can check to see if there is anything drawing the battery down when the key is off....just a few things to start with....some of the smart guys will show up to really help you.