I think you have several things going on:
First, if you hold ONLY the Display Selector toggle down for a few seconds, the gauges will all do that dance you've described--it's like a diagnostic routine.
Second, to calibrate the speedo, you ONLY use the speedo toggle while running the boat. So if your GPS says 21 and the boat says 19, then you press that toggle UP. If the speedo is faster than the GPS, toggle down.
Now, what I suspect may have happened is that you held the speedo toggle down for so long, it's at the zero mark. Try running the boat and then toggling up to see if you can get the needle to move.
"This one goes to eleven." 2007 ProStar 197 TT w/MCX and Powerslot.