duff speedo

HI I have a 306 xsi aith a duff speedo. Sometimes it will behave for weeks on end then it will play up for mins, hours and sometimes days. There is a temp cure that works sometimes and that is to turn the lights on and off, but this is not guarrenteed. The garage has changed the plug at the back of the head but still no luck, they say I might need a new head!!!!!!!!! Any ideas please Dave snipped-for-privacy@scubadivers.co.uk