connection to the back of the alternator loose, alternator loose, belt
slipping or alternator becoming faulty.  And maybe alternator earth link
loose.

The connector is a spade one, goes to the W terminal, IIRC.

What Austin said.  On mine it was just full of mud and crap, but had
also almost cut itself off.  Note that it's a short wire and attempts
to solder it or shorten it so it fits better will just make it work at
first, but break within a week or so ... ;)

I ended up replacing the length of wire with a new spade connection and
the old one cleaned up as best I could, never had a problem since.

As Austin mentioned, it could also be a pre-cursor to total alternator
failure - it happened to me on the way home on a trip to Harrogate from
Somerset - occasional wobbly rev counter for about 200-300 miles, then no
rev counter at all, then progressive electrical systems failures - radio
first, then instruments, finally speedo, (no battery light came on though).
Finally let go completely just south of Strensham services on the M5.

I hope it is a straightforward connection issue, otherwise could be quite
expensive!

Tim