My jeep has a idle/start problem.?

I drive a 1999 Jeep Wrangler TJ 4cyl manual transmission. Sometimes when I try to start it, she cranks until I rev the throttle a little bit and then finally she turns over. If I let go of the gas, it promptly dies, unless I've been moving down the road for about 300ft. She idles at a real low rpm unless I rev it at a stop or while moving in neutral. She runs fine otherwise. It seems really weird. Neither I, or even a mechanic, could find the problem with our quick searches. Any suggestions?