Jumpy Speedo

Another new problem in my CLK. I hope to avoid going to the dealer again (the less it is there the better)
Once I reach about 90km/h the speedo stops moving up smoothly (it moves in bursts of about 5-8 km/h) even though I continue to accelerate at a constant rate. Below 90 is ok. It only does this as speed increases, not if speed decreases, that is it will decelerate smoothly from any speed.
I have gone as high as 160 km/h to test (haven't been on the hiway, only city) and it exibits this behaviour consistantly above 90km/h.
Any ideas ? Could some piece of dirt of something have fallen in there somewhere ? The dash was recently replaced at the dealer.