A diagnostic scan would be very helpful here, but in the absence of this, I'd suggest you start by checking the wiring and connectors.
The basic steps would be to start in the passenger footwell and chech all the connectors are secure and the wiring is in good condition. Then check the speed sender, which is at the back of and directly attached to the gearbox (basically, whip off the bonnet and check as per the passenger footwell wiring and connectors). Finally, check the connectors and wiring at the back of the instruments.
If these prove to be sound, then I'd suggest the speed sender has gone, but this should be checked prior to fitting a new one, as they're £45.
Cheers,
Mike