1999 gmc jimmy starting problem?

ok a few months ago i went to go start me car to go to work and when i started it, it only clicked it didnt crank only clicked and it sounded like it was coming from the starter and i put a new started in about a year ago but i pulled of the tire and looked at the started the other night and one of the wires that is on the bottom that is going to the positive battery cable looked all corroded and what not but the thing is is that it only sometimes does that most of the time it started right up so if anyone can help it would be greatly appreciated.