Intermitent speedo

I have a 306 xsi 2lt, every so often the speedo stops working, the only temp cure I have found is to switch the lights on then off again. If the lights are on when the speedo stops I have to turn the lights off then on again. I have taken it to a few garages ( non Pug ) but they either don't know the answer or suggest a new head!!!!!!!!! any ideas would be appreciated Dave