There was a thread a few months back where the OP fitted 4 new thermostats before getting one that actually worked properly.

https://www.jaguarforums.com/forum/x...ue-not-279115/

 

Interesting, I wonder which brand he used in the end. The one I took out was a aftermarket brand that I can’t remember, I put in a Waxstat which seems to be the one everyone recommended but now I’m having the problem. Might put the old one back in for a while and see what happens. I replaced it to try to get fuel emissions to improve, but that required a different solution so it was likely replaced for no good reason.