So last year we purchased a 2001 XJ with 4.0L. After owning and driving for about 1000 miles it developed a problem where it suddenly starts losing power and running rough. This doesn't happen immediately, but seems to occur after 5-10 minutes. It runs like a champ and then starts to lose power, sometimes surging as if you were stepping on the gas pedal and letting off quickly several times. The Jeep starts to vibrate as the engine feels like it has lost a cylinder or two. Backing off the gas and trying to keep it moving slowly and eventually it runs normal for a short time before acting up again. We have replaced the coil pack, spark plugs, and fuel pump. The check engine light doesn't work, even running the IP test, but there are also no engine codes when looking at it with a code scanner. Not sure if it is a sensor or what could be causing the issue. Has anybody else ever experienced a problem like this?