In the last few days my 02 xj8 with 80k miles has had a very erratic temperature gauge. The needle sits on the bottom most cold hash mark for longer than normal then shoots to the middle of the gauge within 20 seconds. Then it'll hold steady there for a few minutes and within 30 seconds it'll shoot up all the way to the hot end of the gauge. The car is topped off with coolant so I'm thinking the gauge is going.

Two other (maybe) related things are I've noticed the fan kicking on a lot for the past two weeks. I never used to hear it, now it's on 50% of the time I'm driving. Also for the lat year the car has given me low coolant warnings almost all of the time even though it's topped off. If i pull the cap off the reservoir and put it back on the warnings go away. This probably doesn't matter, just thought I'd mention it.

Any thoughts guys?

 

Thermostat, betcha. You want to get this fixed asap, you are risking your engine when it gets to the top of the scale.
Do a search, there's plenty of posts here on how to change it.

 

Yup, he said it. Your thermostat is sticking. Change it ASAP.

 

And make sure you get one from a normal part supplier for $4 vs the $40 my Jag dealer wanted!
It's the same thermostat as a late 90's Thunderbird Super Coupe if I remember right. It was many years ago I had to change it...

 

In addition to the thermostat, make sure that your cooling system does not have air pockets in it. You will need to burp the system.