[Saurian]

For reference, I was informed that the gauge on the XJs is not quite right... It sits there, innocently just slightly above 210, and you think all is well, till it blasts up to 260. The engine is not just suddenly going up 50 degrees, for some reason, the gauge does not register that range; instead of sweeping up, it hangs at 210 till it gets past the overheat mark.

As far as initial cause, I can't help you there; it's been four years or so since I sank mine in the mud, and it's still running a teeny bit warmer than I would like it to be... Then again, my radiator is 8 years old...