1999 Jeep Cherokee Hard time getting car to start

Have a 99 Cherokee and over the last several days it sometime won't crank. It turns over no problem, but won't crank unless I give it gas. Then I have to keep it at about 2000 RPM's for about 45 seconds or so so that it won't cut off. I find that if I throw it in gear real fast I don't have to keep it revved. Once in gear it runs with no problems. Twice now after trying to start it a couple times you go to turn the key for another try and nothing happens as if the battery has died. You wait a minute or 2 and then it turns over again but doesn't crank. Took it to a mechanic and he is unsure. Of course it wouldn't do any of this for him. Any ideas?