Johnny5isalive

I have a problemo with my98 uk speedo,every other time i start up and set off in my car the speedo needle just stays put on its rest,ive discovered that slapping the top of the clock cowling frees it and it resumes an accurate reading of my speed.I cant imagine its an electrical problem as it would be more intermittant.One time i was doing 70 on the mway when the needle started going round,i watched it go past 160 then got stuck underneath its little rest,i had to remove the clear front casing and flick it back over.Anyone else had this problem,or know a fix? I wouldnt even know where to start if i have to take the clocks out