The grey MK1 2.4 speedo was bouncing a little up to 30 miles per hour but beyond that it really got bad with swings of large variations which meant that at 40 MPH it would flick up to 60 MPH and back again. It looked like a crook speedo cable but as it had a brand new cable I suspected the instrument.
I removed the speedo and on the work bench connected up a drill set to run in reverse with a bit of old speedo cable in the chuck of the drill. I found that the problem was in the instrument as with the drill full on I got 45 MPH with bouncing up to 70 MPH.. I attempted to remove the speedo case and to repair it but was unsuccessful as the trip odometer connection was jammed.
Luckily I have accumulated a number of spare MK1 speedos and I had one which matched the diff ratio for a 2.4. I checked it on the bench and it worked OK but I noticed some serious lag in the indications as it took 5 seconds to catch up to correct speed and a similar time to come back to zero after the drill was stopped. I removed the case and delicately applied with the smallest artists brush a dab of WD 40 on both jewels [pivots] of the speedo needle. That fixed the problem and the replacement speedo was installed and is working as Jaguar advertised.