Machine is an '04 Sp700 carb'd and the only changes I made before the speedo decided to bug out is unplugging the fan to wire in a on/off switch because the fan would not turn on by itself.

Keep in mind for the entirety of this video, the machine is standing still.

You can watch it be totally wacky here:

YouTube - 2004 Polaris Sportsman 700 Speedo

 

Maybe dirty Hall Effect sensor,Part number #27. I say dirty because usally when they go your speedo does not come back to life. If your AWD still works but speedo does not then the speedo is good but its wheel sensor(Hall Effect senor $45) is bad .

 

looks almost like the tac working instead of the speedo. did you unhook the switch you installed and try it again to see if it worked normal? if it does work then you know you got some wrong wires some where you tapped into. just a thought

 

A wheel speed sensor does make sense, I'm gonna check that out this week. Thank you very much!