Had the same prob with my 99 megane, i found that corrosion had eaten through one of the wires, causing intermittent problems on the speedometer. Best place to check is the main fuse relay (its the black box thats in the same compartment as the battery) open it up should be 4 or so fuses and a white block connector. check to see all the wires are ok, i think the speedo wire is Grey. if it has been corroded just solder the wires together. hope this helps
william
May 2007