If you have the 4.0L SOHC, it is most likely the cam tensioner. There is a known problem with it. I had the same problem also. Sometimes the ticking would get really load and the idle would drop down to like 500 RPMs. I took it into the dealer after reading all the posts on this board about that particualr part being defective. I told the guy it was making a ticking noise and the cam tensioner probably needs to be replaced. He kinda blew off my sugestion and said "awww, we've replaced the cam tensioners before and it hasn't made a bit of difference.". Keep in mind this was just the guy at counter. I called back the next day to see if it was ready and he said no, that a technician hasn't been able to look at it yet to determing the casuse of the problem. So I call back the next day and it was ready, and they did indeed replace the tensior with a revised version.