your radiator is now 15 years old and needs a recore
they wear out and sludge up
this guy's brilliant website shows what yours looks like inside
http://bernardembden.com/xjs/clograd/index.htm
you should get a full recore for $ 300 or so
This is the first independent UK classic car club totally dedicated to the Jaguar XJS and all Jaguar XJS Enthusiasts out there around the world WWW.XJS.ORG.UK