The same situation happened to me about a year ago. I thought it was my subwoofer overpowering the battery/alternator and got rid of the sub and replaced factory alternator w/ high output.

Then a few months ago, a guy from a shop drove it up to the lift and said everything just died and at first he thought it was really wierd. then he jiggled the battery ground wire bolted to the body and it was loose. Even though it was bolted on and you would think it's on good, just a little play makes a diff. The guy told me to bolt the ground to both the body and the engine block and no problem since.