Very common problem (or so I'm told) in that era Falcon. I had a similar
drama with my ute. The most likely cause is a little plastic cog type
thing in the back of the instrument cluster. Impossible to fix yourself.
The only thing you can do is remove the cluster and take it to a
instrument fitter. Don't even bother with your mechanic because he'll
have to do the same thing (and then add 20% LOL).

JC