I'm not sure if anyone asked this or not, but I was wondering. How old is the Battery? I have a 94 Chevy pickup, and I bought it brand new, and after it's first battery went bad, (Around 4 years) it would do the same thing. As long as I drove it all the time, it was fine, but drive the wifes car for a week or two, trucks battery dead as a doornail. I'd push the truck down a hill, pop the clutch to start it, and it would charge back up and be fine, until I'd park it for a week or two again. Finally, I just got a new battery, and all that stopped.
Just a thought, and even if your battery is new, it could be defective as another user mentioned above.
Good Luck with it,
Sunday, April 20th, 2008 AT 9:13 AM