97 jeep cherokee stalling

I purchased a 97 Jeep Grand Cherokee, 4.0 liter engine, with 85 thousand miles on it one month ago. After one week we had problems starting it. It would turn over fine but would not fire. After several tries it fired. The past few weeks it stalls or will not start right away almost every day. We have taken it to the dealer on several occasions. The first time they could not find any fault codes. The third time in they found a code for the crankcase sensor and they replaced it. We have also had the throttle body cleaned and a wire to the PCM replaced. The battery has been tested and is strong. The car still stalls and sometimes will take five minutes to restart. Help!!!!!!!!!!!!!!!!!!!!!!