The 4.0, especially in the xj, is well known for this. It is caused by the #3 injector being located extremely close to one of the exhaust manifold pipes. In this area, it is exposed to alot of heat soak. This can lead to the fuel inside the injector vaporizing, leading to a misfire condition due to a temporary lack of liquid fuel flow. Obviously being exposed to higher than normal heat can lead to premature failure of the injector as well. I'm sure having to deal with this vaporized fuel condition isn't exactly easy on the injector either.. A good way to counter this and prevent it from happening is to get some of that quality heatsheild tape (available at most auto parts stores) and wrap each injector with it, like a mini heat shield. This can help lower the temperature enough to stop, or at least minimize this condition.