unfortunately the obvious solution of replacing the speedometer is not the solution. Replacements have been made and it does not resolve the problem the culprit is elsewhere, most likely 1 or more of the sending units that provide information to the instrument cluster. Sorting that out and the somewhat randomness of the problem is probably making this take a long time to resolve. I am not defending triumph per se, just saying the obvious isn't always so.
Hadn't consider that it might be elsewhere, but it make sense.