Jeep with serious problems

Okay, I just bought this 1999 jeep cherokee 2wd 4.0 v6 and Im very upset with it! To get to the point, Sometimes It will start automatically but most of the time it will crank but not fire up. It will crank on and on and still nothing until finally (after 20 minutes of trying to start it) it starts. This has been going on for about two weeks now. What concerns me is the other day while driving and on a date my jeep completely shuts down! Out of nowhere I tried to press the gas and it wasnt even on! So then, I pulled over to try and start it and it did that same hesitating problem then started. My mechanic is sort of stumped with this aswell, He said my fuel pump is turning on and it seems fine, we both have had so many hypothesis and we just want to get to the bottom of it. Just yesterday my check engine light turned on soo today hopefully I can take it to auto zone and they can shed some light.

I dont know if this is related but when I have my air con full blast it seems as if my car runs very sluggish, Hesitates to accelorate and the fan sounds horribly loud in 1st and 2nd gears. Is this normal? What is causing both these problems. Please someone help!