I had the exact same problem, it used to scream at me over 60 mph so I phoned up for a new replacement -?115, so decided to strip it myself.
I took the dash and instrument pod out. disconnected the back of the pod and unbolted the speedo. It has some form of disc in a cup that needed a clean out, put it back together and it has been fine since. took about 30 minutes from getting in the car to test driving it, so it's not a big job. you just have to remember where each bolt,screw, and cable goes when putting it back- save yourself a few quid too.