What's wrong with the Speedo on my Ford Focus?

Hi
I have a 2004 Ford Focus. For over a year, I have had an intermittent problem with the speedo, in so much that all of a sudden it will just stop working, but then magically it will just start.
I took it in last year to get it repaired, but at that time it was working, they took it for a drive and couldn't find a problem. Yes, you've guessed, I picked up the car and after 1/4 mile it stopped again. Again I took it in and they said that until it stops completely, they wouldn't know which part to replace, as if they went for a more major part it would work out dearer and may only be something cheap. Any ideas would be appreciated.