The suggestions above are all good - I went through those a few months ago. My issue with your same symptoms was a loose ground cable at the battery terminal. In fact, while it appeared fairly tight, the clamp (off the - was not stock) was cylindrical, whereas the battery terminal is slightly tapered. This made for a poor/loose connection that miffed me in what was going on (had the car towed out of my garage to the shop, only to start off just fine off the flat bed). A quick check; just my 2 cents of input.