I don't seem to have these problems, other than the AC thing. Mine seems to be the wire into the fuse block.
The XJ6 is an excellent car. I've had mine for 6 years now and I plan on keeping it a lot longer.
I have to agree the XJ6 is an epic car, but i'll tell you what I done with mine. I raced it as a banger on a tarmac oval track in the UK. It looked the part when I turned up at the track with my XJ6 on the back of my hiab, it also performed on the track. I had a Ford 2.8 V6 motor fitted to give more hitting room in the front, I followed about 3 people into the fence and reversed out again and carried on going, that's just how strong the front end is. It lasted all day until the DD, where I got put away in the fence at quite a speed, the back wheels were up behind the drivers seat nearly and there was a clear print of the front of a Granada in the back of it. All in all the XJ6 is an epic car, but in my eyes is better off on the banger track!!
I have had mine since 1997 and it been pulled apart and lots of cash chucked at it and everything works great. I would say most of your issues are age/maintenance issues. Th window switches are easy to fix, pop them out and give them a clean up. Great car that's why I keep it.
I have an '84 Sovereign with some similar problems. My speedo stops working sometimes, but I'm begining to think this may be heat related. The cruise control is weird in that it takes a few minutes to start working after I hit the button, when it does actually work. I'm also having some real problems with the headlights, when I turn on the high beam only one of the centre lights comes on and one of my headlights drops from drive to park. Any hints on these would be great as I can't find an auto electrician who knows what he's talking about.
I've owned XJ6 Jags for 15 years, only have 1 now.
The lights problem is easy to fix, open up the bonnet, just in front of the battery near the wing/bonnet divide is a fuse box, take each fuse out and clean the fuse and the contacts and all should be well. The speedo problem is usually the transducer on the gearbox get wet and dirty, again, get under the car, the transducer is easily located near the back of the gearbox, remove the connection and clean. If that does not fix it, fit a new transducer, not very expensive.