my jaguar convertible xj6 seems to have the exhaust plugged up and overheats. what could be the problem and solution?
Any CEL and codes?
A clogged catalytic converter or crushed exhaust pipe or muffler.
Check engine light should be on if the converter is clogged.
How do you know it (exhaust) overheats?
Not if the oxygen level of the outgoing exhaust offers a sufficient change in the oxygen content over the oxygen level of the exhaust stream going onto the converter. 
I too agree with a plugged exhaust, possibly a collapsed ceramic honeycomb in the converter core, or a somehow plugged exhaust pipe.
But if that doesn’t pan out, a lean fuel metering is another possibility. Just as using a bellows in the fireplace makes the fire go hotter, fuel metered with more portion of oxygen than there should be will heat up an engine’s cylinders.
The solution depends on exactly what’s found to be the root cause. If it’s the converter, a new converter is the solution. If it’s an exhaust pipe, a new pipe is the solution. If it’s a lean mix, replacing the cause of the lean mix is the problem. Unless the lean mix is being caused by a vacuum leak, in which case replacing the vacuum line is the solution.
Can’t offer a solution until the problem is diagnosed. Sorry.
Convertible XJ6? What year? Don’t think they ever made one. edit - nope, never from the factory.
Guessing here , I think the OP had two XJS Jaguar convertibles before the purchase of this XJ8 that had transmission problems and said they were going to sell it. A good muffler shop could probably solve this current problem. The other thread ran to +70 posts, lets hope this one does not.
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Are the exhaust manifolds glowing red? If so, that usually means clogged converters and a BBQed engine if this goes on very long.
Based on your symptoms, I’d start by checking to see if the exhaust is plugged up. I’d start with the muffler and then look to the catalytic converter.
Thanks for the specifics
Lots of good directions to follow! Really appreciate all this though I don’t understand much just the big words to follow