Originally Posted by MeefZah
This is a problem with (at least one) WR450 also, my riding buddy's speedo went apeshit in much the manner you describe, reading crazy high. In his case it just happened, there was no dead battery or other event to point your finger at.
Looking at the 250 and 450 components, they are very similar.
You may want to expand your Google search for solutions to include the 450; though I don't think he ever solved the problem.
Everything I've found (what little there is) is 'replace the head unit'. I'm replacing the battery 'just in case' this weekend (and with a Lithium one, in the hope that it will ALWAYS either deliver solid voltage or no voltage, like every other lithium I've ever used in other devices). If not, it'll be new head unit time.
I don't tend to keep the WR on a tender; it'd been fine and doesn't actually even have a pig-tail installed. Be changing /that/, I can tell you.
Still a great bike. Just no longer as cheap as it was.