My guess is the battery isn't holding a charge or the alternator isn't charging it enough to crank it over. I had the same problem a few months back. I first got a new starter. No change. New battery came next. This solved the problem for a month. Then the problem returned. I took the battery back and got a new one. About 2 weeks went by and then the problem was back. An alternator from a junk yard finally fixed the whole thing. Turns out that the battery went bad from being undercharged by the alternator. Funny thing is that my voltmeter was saying 13 to 14 volts as well.