No sh*t Sherlock!
Its obviously an electrical fault, but where do I look?
well my thinking is, if its a duff sensor, it just won't read right all the time.
if it works for a while then not, my guess is something sin't connected right.
my advice get the service manual out and track back the speedo wiring. i'd start in the fuse box first just to make sure it isn't a loose fuse.
simplest answer is usually the best.