The pot is easy to check with a VOM; just make sure it provides a smooth resistance change from idle to full throttle.  If not, replace it.  It is possible to rebuild it, if you're the kind to rip into such things.

If replacing, good news: later XJ-S's came with a much better pot made by Lucas (!) that costs a bunch less, too!  The only problem: it requires buying a couple of other parts to be able to use.  The total cost is still less than the original Bourns pot, that's not the problem; the problem is that Jaguar won't sell it to you unless you ask for it!  If you merely ask for a replacement pot for your car and tell them the year, they will happily sell you the overpriced unreliable Bourns pot without even telling you there is a better option.

Jon Heflin found all this out, then provided the following illustration of the part differences:

You don't really need to know the part numbers; just go to your local Jaguar dealer and insist upon the "red pot" and the necessary hardware to mount it.  Their fancy computers will tell you the parts you need.

Greg Meboe provided a picture showing both pots installed in their support brackets.

The Bourns pot is the black one on the left, bolted directly to the bracket.  The better Lucas pot is the red one on the right, installed on the same type bracket with an adapter mount.

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