I recently purchased a 1998 XJ8 with about 45,000 miles. It ran fine for the first little while but recently it has just cut out while driving. Both times was on the hiway and it felt like it wanted to refire. She pulled over and after a little bit of cranking it starts up with a cloud of gray smoke. The first time it took quite a bit of cranking and the second it fired almost immediately.

It was previously owned by a 90+ fellow so I don't imagine it was driven hard.

Any suggestion on what it might be and how to check or fix it.

 

These symptoms were commonly caused by the throttle body; there was a recall. A dealer can check your VIN and ascertain whether the recall was done.
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I believe the throttle body has been replaced once previous to my buying it. If it is the throttle body do you know where I can get one a a reasonable price/

 

this is not usually a throttle body recall. Your issue is usually a clogged part load breather(small plastic pipe of the drivers cam cover) carefully disconnect it, and clean with a small drill bit like 1/8" or alittle smaller. Also clean the throttle body with throttle cleaner and a tooth brush. pay attention also to the port in the throttle bore for the idle air control. If you issue is a steady cruise or youve let of the pedal to decel like for an exit or traffic and it stalls then this is your problem. the throttle closes and cannot bypass enough air to keep the engine running. you pull over of throw in nuetral while coasting and it restarts