I have a 2006 Grand prix base model with a new battery I put in back in November. Intermittently the car simply will not turn over, so I get it boosted, the car is fine for a while, then it won't start again...rinse and repeat. Today I put a battery tester on the battery and it showed 12.7 volts not running and 16 volts running. The battery is being overcharged obviously, but why? and how can I test to see if its the voltage regulator or something else.