Won't always start
 

I have a 91 S2 new recent factory battery a rebuilt starter a year ago. I will drive for any amount of time when i go to start it it acts like the battery is dead this happens randomly. I have checked all of the grounds could it be the starter?

I could be the alternator not fully charging the battery. Mayby the alternator is going bad.

bad/corroded connection at the started, faulty ignition switch, bad relay. electrical gremlins suck.