I've been having trouble for a while now with an intermittent speedo in my 1988 XJS (electric not cable speedo). I changed the speedo a while back and thought I had fixed the problem but that was just luck. Turns out it was still intermittent.
After mucking around under the dash recently to clean the AC drain I've made it worse and now it hardly ever works.
I suspect one of the plugs into the instrument cluster may be dodgy or maybe the printed circuit of the cluster is damaged. Everything else works fine and I runs an additional earth connection.
Anyway, I've always disliked the XJS instrument cluster and would love to upgrade it to the mid 90's facelift cluster. From pictures I've seen they appear the same size and shape and even have 2 similar looking plugs at the back. Is it a straight sway or is there a fair bit of re-wiring required? I'm happy to do a bit of re-wiring but don't want to go too crazy with it all

 

Not sure on the fit and forget aspect of the upgrade. It will be "not so easy" that is a surity.

Have a poke around the LH side of teh console cheek, near teh fan relay pack. Somewhere there ypu will spot a Yellow wire. It is the signal wire from the transducer, and in that area it splices off to the speedo, trip computer, cruise control. My '85 had issues with teh cruise, and I found eventually (it took many beers) that the soldering of that splice was crap. Once sorted everything worked properly, but the cruise eventually died of non related issues.

The circuit, maybe, but grubby contacts in those 2 plugs is more likely. I clean them with Lemon juice (the real stuff), and carefully rinse them out after, and apply a smear of Silicone grease (plumbers shop), and that has worked for me.

 

You could always try to find the old auto style conversion. Plug and play.

http://www.jagweb.com/autostyle/index3.html