I have a 99 Cherokee Sport, w/straight 6. Runs

I have a 99 Cherokee Sport, w/straight 6. Runs like a champ when driving but starting it usually takes 2 tries to get it to start. If you start it within 5 minutes of shutting off it will sometimes start first try though. Also rough idle for a minute or so after getting started. Also notice that when I run shell premium sometimes it will start first try especially when fuel tank is full. Plugs are good (Bosch Plat. +4) any other ideas? Thanks!
Leann