2000 Cavaliar won't start

I have a 2000 Chevy Cavaliar, it has 65,000 miles on it, and it starts when it wants to, and there's no certain time when it won't start. So I replaced the alternater, the battery, and the starter. But it still won't start most of the time. What else could the problem be? I've checked the ingnition fuses in the car and under the hood, and I've also checked the battery cables to make sure they're not lose. And when I attemp to start the car it makes a hissing noise.