randomly won't crank

My 94 jeep cherokee 4.0 randomly will not crank. When turn the key I can hear the Starter relay clicking but then nothing. The nuetral safety switch has been replaced in the last 6 months. I took the starter off to replace it and took it to Autozone and when they tested it, it worked so i didn't replace it. I cleaned up and put it back on. After repeated trying and waiting it will start. Is this the ignition switch? Or is the starter going bad? When I test the voltage at the battery terminals and it is 13+ and the connectors are tight and i have cleaned them both. The starter has 13 volts when the key is not turned between the top terminal and the ground terminal. Any ideas on things to try? I would take it into a shop but its a random problem and I really can't afford for them to just start replacing things. Thanks in advance!

Bench testing, especially for an intermittent problem, may not be conclusive. If you can verify that you have 12 volts at the starter WHEN the no-crank occurs, the problem is the starter or solenoid. Sometimes tapping on the starter with a rubber mallet reveals the problem as well.....

Starter Relay??? Easy JY swap and they prolly won't charge more than a buck.

The first thing I always look at is the brushes. They do wear down over the years and/or get covered in oil (or worse) on occasion.

http://www.go.jeep-xj.info/HowtoStarterClean.htm