This sounds like an intermittant faulty connection which is causing spikes of power, which upsets the electronic ignition and the speedo sensor. Is there any sign of a dull sidelamp? - or any flickering (best noticed in the dark). Are the panel lights working OK??
I would start by cleaning the sidelamp fuses, and the contacts in the switch, since this is easy to do. Beyond that it gets much harder. A bad earth is often to blame, so check the sidelight earths. My gut feeling is that arcing at the switch could cause the speedo flutter, but not so sure about the misfire under load. That's an odd effect.
Arcing can cause a gunpowder smell, so sniff around the steering wheel cowl and under the dash. Good Luck.
