I have a 98 xj it only overheats when I'm driving for an hour or more at speeds 55+ and only when it's warm out. Any other time I'm driving it stays at operating temperature. Also I've noticed the coolant overflow resivor will slowly go down over the course of time ( the more I drive the faster it goes down). There is no sign of a leak anywhere. It has a new head gasket and brand new head. I've also replaced the Thermostat, heatercore, and the Radiator along with the upper Radiator hose. The only things I haven't touched are the water pump and the lower Radiator hose. My Thermostat is a 195.F. any ideas people?

 

see what happens if you turn on the heater with fan full blast

if that alleviates, points to inadequate radiator flow

If not, I would change the water pump next, they are cheap as dirt

 

Agreed on that strategy. Unless the heater core is clogged, of course.

 

You don't mention flushing the system, but it may be time for this: