97 XJ 4.0 166k miles. When I first bought it I did a tune up (3k miles ago) with autolight copper plugs (I know everyone say's champions), new wires, cap, rotor. #2's insulator was broken in half. I did not like this but what can you do. Anyway it was running fine, every so often CEL would pop up but ran okay so no worry to me. Fairly certain it was reading multiple misfire and cyl # 2.

Today on my way home CEL was flashing and it was sputtering really bad. I checked codes and cyl 2,4, and 6 were misfiring. I decided to change those three plugs and when pulling #2 I saw a hairline crack down one side of the insulator. I replaced all 3 and disconnected battery for some time. After doing this CEL is gone and it runs fine again.
I know it has never ran perfectly, but I'd like to figure this out. Getting 10mpg leads me to believe there is some sensors bad.
I've read it can be due to heat. My exhaust has a hole in it, and I was waiting till spring to fix it. I'm sure that could be part of my problem with o2 sensors and everything.

Maybe I also have bad injectors?
Any help would be appreciated, thanks.