Only twice in over 21/2 years of Lancer ownership I have seen my speedo exhibit similar symptoms. However, they have been isolated incidents and have never led to any other problems.
In my case the rev counter and speedo dropped to zero, then flicked back on and off a couple of times before returning to normal. The only thing I can tie this behavior into is the failure of my idle control stepper motor (still needing fixed!), but thats just a guess.
This happened to my car shortly after taking delivery. To drop the speedo from kph to mph, a small 'gearbox' is used to slow the cable down. In my case, the nut which held this box onto the gearbox drive had come loose.
I noticed my tacho have an attack once and bounce around a bit even though speed was constant (on motorway). Any ideas why? Doesn't bother me much as the car was running fine and still is. Just curious
You will probably find its the speedo convertor (if fitted), it is usually fitted on the gear box speed sensor (grey or black box) to check you can remove and fit back to standard, this will now read in kmh.
This is what is fitted to a official evo.