temp is fine when running down the road, but when I'm in stop and go traffic it starts heating up, I can put it in neutral and rev to 2000 rpm and it will cool back down.
I have changed out the water pump made no difference.

 

It is likely not the water pump. The thermostat and thermostat housing are known to come apart and block coolant flow. I would order the thermostat and water outlet assembly and replace everything as a unit. That is what I did to resolve the exact symptoms you are describing, as long as your cooling fan is working as well (mine was spooling up to max speed).

Dave

 

I agree on checking the thermostat housing. If you pull just the small elbow at the center where the thermostat is you will see if its broken down inside. I would assume you had it off to do the water pump, but perhaps not, or maybe its been a while. Jaguars fascination with plastic bits for engine parts has often proved to not be the wisest way to sane weight and or cost. The mostly plastic secondary chain tensioners on the 4 liter engines, the thermostat housing on those as well - they came up with an all metal housing for the 4 liter but not for the 4.2 that I am aware of.