Car don't start then does

I have a Hyundai Elantra 1997, 4 cylinder w/ 165000 mls. I have replaced the starter, alternator, and battery 3 years ago. About a week ago I started too notice my car was taking longer to start then usual. Three days later I would not start at all just a click sound, bought a new battery even though all the lights came on, tried too start it with the new battrey and nothing just a click sound. Went to start it in the morning and it started up, althought with some delay. Took it to the mechanic they tested the starter, alternator and battery. They said they noticed a slight glitch with the starter but did not think that was the problem. I left there with the car starting up WITHOUT delay. That was 1 week ago. Yesterday while driving around I noticed my car started taking longer too start up again. So I took it home tunred the car off and waited about and hour and went to go start it up and nothing again. Went to start it up this morning and it started up with just a slight delay. I'm so confused, the mechanics say they don't know it could, I've research on the net, but have not found cars starting up fine then not, back too starting up again. Please HELP!? ?