not firing properly.

my 88 xj 4.0 has 140k on the clock and bogs sometimes. Anywhere in the rpm range it will sometime lack power and sound a bit odd-like timing maybe? It gets a little jerky, and doesn't ever kill the engine, just sounds odd and lacks power. Then it'll stop and the truck feels like a rocket again. It is modded, but it has done this for a long time, it's just getting more annoying. Any thoughts?

what do you mean by "modded"?

it's more than likely NOT the timing, 4.0's only with the distributor in one position

http://www.angelfire.com/my/fan/dist_index.html

make suer that the resistance of your temp guages is correct, they caused me troubles and were hard to pin down as the problem because they changed resistance every time they warmed up.
Just a thought.