I cannot help with a local repair...but you may have to replace or repair the entire cluster. I just traded in my 03 Silverado and was having problems with the gauge cluster. Did find a few places in the States that can do it for a reasonable price...in Canada they will want to rape you.
Must be common. My 2003 2500 was doing the same thing when I sold it. They had speedo technology perfected in about 1950 I think. To be able to screw it up over 50 years later must have taken several engineers many late nights.