Bad Speedo?

I've read the speedometers on the 2000-2001 Nautiques are known for having problems. Just got the boat out for the first time, and the speedo is reading all sorts of speeds, like it's flickering or something, regardless of how fast I'm driving. At higher speeds it seemed to read ok for a bit. It's a single gauge but displays the speed in two sections on the unit, so I assume one pidot tube is for each speed reading on the single gauge. Neither reads right and are all over.

Don't want to run out and replace a speedo since electrical parts aren't returnable, but it seems that must be the issue, that the gauge is faulty. Anybody have any ideas?