S10 won't start uh all the time

My 1997 Chevrolet S10 Vortec V6 4x4 many times won’t start and needs a jump. Indescriminately it chooses when and where. Two years ago we tried changing the alternator and battery and having them checked at Autozone only to find out the units were fine and we re-installed them.

Six weeks ago we took it to the pros, our local chevy dealer. They told us the alternator was only putting out 8 volts and it needed to be replaced. We gave the go ahead and paid the $389. Friday it happened again and they said they would check it no charge. No charge? Your bleep right buddy and we’re gonna see who’s gonna pay for that misdiagnosis.

Anyway we are looking for someone who knows where to look on the truck for a possible short or something