Is this another "quirk"?

[Sam Gamgee]

Every time I stop (engine still running) the speedo on my 500 starts reading bonkers figures like, er, 70 or 80 mph!!!! Note that, whilst not being over-worried bout this, would like to have idea of problem. Could it be sender, maybe? Anyone else out there with same thing happening? If I rev a little and leave it sorts itself out eventually. Reads ok during driving....

[Wildfire]

Sounds like an earth.