I had a similar situation on my Buick. Thought it was the starter so took it to AZ and they said it was bad. Put the replacement starter in and it started up but next day did the same thing again. I found out the positive cable from the battery to the starter was bad (intermittent). Changed that out with one from NAPA and problem solved until the 8 month old replacement starter broke but thats another story. Try moving the positive cable around when it does it I could get mine to turn over.