I pulled the thermostat out and checked it in a pot of hot water. Seems to open at the correct temp and stay open as long as the water temp stays above 85C. Closed right up when I took it out of the water and opened as soon as it got back into the hot water. The gauge seems to be correct. I drained the system and ran water through it with a hose at the expansion tank, the bleed opening and at each of the radiator hoses. All seemed to flow freely. My next thing to check will be the flow of the water pump. I am not exactly sure how I am going to do that so if there are any suggestions please let me have them.

 

It is still wise to replace the thermostat. It is not uncommon for them to stick intermittently. They are inexpensive parts.

 

Welcome to the forum. If you look at the top left of the page you will see the USER CP box. In there you will be able to put your car details in the signature, so everyone will know what year and model you are discussing.

I have not removed the water pump from a XJ8, but it is very easy on the XK8, and I would expect the same. That is the best way to determine whether the vanes are intact.

 

Especially if your car is a '98 or '99, and you don't know the status of the water pump, I would go ahead and replace it. There have been numerous disasters from water pump vanes disintegrating. It's not a bad job.

 

I replaced my water pump (2000MY) as a precaution - although the original pump seemed in good condition regarding the vanes. Its not a difficult job if the bolts aren't seized - mine weren't after 13 years.
hex sockets always - if you dont have any buy some !
The "replacement" pumps have paper gaskets so a little blue sealant is needed at the faces.
Replace the thermostat while you're in there.
If guage still playing up look at the temp sensor at the top hose plastic tubing.

Regards
AndyP