Crank Shaft Position Sensor? or what?

I've had my xj for about three months and have done a lot of work to it.
I changed the timing chain when i first got it and ever since then it has been
chugging like the timing is off.

I read recently that you cannot change the timing on a 4.0, and that its a sensor problem.

i was wondering if anyone else has had this problem before or knows how to fix it...
thanks

Had this prob once and found that the hold down clamp broke letting the dis move around.

Base timing on the 4.0 is not adjustable; that is true. Any check engine light?? What year is your XJ?