Hi all need a bit help & advice.

On the way home from ford fair yesterday my speedo started playing up?
It was showing on 110 instead of 70? on average about 40mph out & bouncing
This has never happened before I pulled on the services to have a quick check on everything but all seemed fine, set off again & it started again??? If anyone could shed some light on this I would be grateful.
It's a staph cos 4x4
Cheers Lee