My car will drive fine for a month or two with no problem and all of a sudden when I turn the engine off to go the store or something it wont start or turn over when I try to start it. After about 45 minutes to 5 hours later it will crank by up. What do u think is causing my 03 xterra to do this?

Optional Information:
2003 Nissan xterrra v6

Already Tried:
Everytime I take it to the dealer they tell me they cant find no problem because the problem needs to be duplicated.