I've had similar issues, all due to moisture getting in the speedo. Can do all sorts of weird things.
Curtis at Wild Steed Worx offers a complete speedo "overhaul" where you send him your analog OEM analog speedo and then he completely rips it down and does all this:
Break Down Speedometer
Clean all internal components thoroughly
Trim excess tips off soldered areas that promote corrosion
Apply new layer of urethane sealant to electronic board
Install watertight, metal strain relief for wiring
Seal front lens to prevent leaks
Install full O-ring seal under bezel to help keep unit sealed
Install new odometer trip reset button, if needed
Purge speedometer with nitrogen before final assembly to prevent the dreaded fogging
I see he has a price of 150 for all that. wildsteedworx.com
You can also buy the digital speedo for the bike. Comes in both blue and red. here's a link for the red one:
New Dakota Digital RED direct replacement speedometer for all Big Dog Motorcycles models from 2004-2011. The speedometer illuminates in RED. The red one tends to wash out in the sun and it's harder to read, the BLUE one is recommended because it is easier to read, but if you want RED it's your...
www.bigdogpartskingpin.com
Happy riding