The same exact thing was happening to me for the longest time. I have a 90 CRX Si with the stock motor and I couldn't figure it out. Well, I eventually gave up and brought it to the mech. He told me it was an electrical problem. Try taking it to a mech and just have him diagnose it. He'll usually let you know what's wrong and won't charge you for it. Having him fix it is a different story...