96 grand cherokee cranks but wont start and stalls sometimes

I have a 96 grand cherokee that sometimes decides not to start. I can drive it for a month or two with no problem and then one day i will try and start it and it cranks but won't start. Sometimes it might take 2 minutes before it finally starts and other times it might take a few hours. Then it could be fine again for a while or it might do it again a few hours later. Also there are times when I am driving that the rpm's will drop to zero but the car is still on and a few seconds later it will catch and be fine. Other times it won't catch and the car will stall and won't start for 10 min to 2 hours. Can anyone give me an idea where to start?