No idea about the speedo reading 186 at standstill - that does sound like more than a rotor.
flimbar - your speedo reading random numbers on and off then finally dying is classic busted rotor symptoms - it starts reading funny as the little fins on the rotor start to disintegrate until they break off completely at which point it reads 0. I guess you might be lucky in between as the rotor finds bits to catch on and give a good reading for a bit.
Easy enough to change - from a parts diagram looks the same as on a curvy so whip the front wheel axle out/front wheel off to get access to the sensor, seal off, then inspect the rotor. Most likely you will have to get all the bits of crap out of the sensor with a screw driver. Replace with part number 34983-21E00 from
http://www.robinsonsfoundry.co.uk/sh...rts/index.html
Plenty of posts on this already as its a common problem.