My 1992 xj Laredo has a jumpy tach at highway speeds with constant throttle. Basically the computer is constantly shutting off fuel to the cylinders (no misfire though) which causes substantial power loss on the highway and slow steady overheating under load. I'm positive it is not a head gasket for various reasons, specifically it is not affected by engine speeds, but it appears as though air flow or vehicle speed. I can pull over and rev the motor to cool it down. Yes, my secondary fan works, new cooling system down to the bolts holding the water pump on. Fresh coolant, possibly clogged but I'm not sure how to check that. I have flushed the system thoroughly. New fuel pump/filter, cleaned fuel system. New TPS. I'm down to idle air control, CPS, or just a new computer. I have a feeling it's the computer but I'm just poking around here before I dive into that one! Thanks!

 

Lets bump you a bit.
What brand TPS? Not sure if your year (but definitely the older years) also need the TPS adjustment.