When I put my key into the ignition and turn it, the car won't start. It won't even attempt to crank. The strange thing is that this doesn't always happen. Four out of five times the car will start. I've had the battery and alternator checked, and they were both fine. The starter was replaced about 4 years ago so I don't see it going bad already. If I keep trying to start it or if I come back and try it again after an hour or two, it will eventually start. The car was been doing this for about six months, and refuses to start on cold and warm days, but lately has not been starting more often. I don't know what the problem could be. Any advice?