1997 Jeep Wrangler Check Engine

My 1997 wrangler with 74,000 miles was just sent to a mechanic for stalling & then not immediately starting. The machanic replaced the cam snensor & I have not had the probel again. But what has been happening is that I had to bring it back to him 4 times for the Check Engine light. Each time the code was a misfire on different cylinders. The jeep is running fine & never seemed to miss. I have noticed that the light came on at least 3 time swhen I was traveling about 65-70mph. All he knows to do is to reset it & doesn't know whay this misfire is coming up. He has checked plugs, wire, rotor & distribitor cap. Do you have any ideas waht could be causing the light to come on like this? The Jeep runs good, but the blight on is aggrivating. Thanks