Intermittent speedo

[monty quick]

My speedo (needle and digital) sometimes drops to zero and sometimes bounces around from zero to the correct speed and, most of the time, works normally. When it is having a crazy moment I am sure I can hear a buzzing which is not there when everything is OK?
I reported this fault when I bought the car 2years ago but Fernhurst couldn't find a problem. It is now getting worse.
Do any of you clever folk have any ideas?

[monty quick]

Thank you clever people. I will have a look this weekend.