Speedometer breaking down?

So yesterday, on the way to work, I noticed that when i got past 70, the speedometer needle started jumping to much higher levels (100, 120, etc). When I was at lower speeds it read more acceptable and reasonable speeds, so it's only past 70 when the needle jumps up so high. I've checked the cable and it seems to be firmly attached to both the front wheel and the back of the mini-speedo, so I'm wonder if it's a symptom of the speedo starting to break down. Unless you guys have some other suggestions?



Here's the speedo I have:





http://www.debrix.com/ProductDetails.asp?ProductCode=39-0550-VT