If the bike runs fine on lower but misfires on higher RPMs, it's most likely an ignition issue.

The higher the RPM the engine is outputting, the higher the current needed from the battery (because the spark plugs have to fire more). If the coil doesn't have enough power, it will misfire (either late fire or not at all).

So I think that either your alternator (or dynamo) is worn out and it takes more power going into the ignition coils than what is provided from the alternator, the battery is dying or your ignition coils/spark plugs are going bad.

You can check the first (and second) item easily by just disconnecting the battery and kickstarting (or pushing it) to start the engine. Then just do a lap and see if the problem still persists (or if the bike doesn't run at all).

The second bit is a bit more difficult to diagnose. I would pull out the spark plugs as soon as you can after this happened (to preserve the state of them) and reference a spark plug chart to see if the engine is running rich or if it's depositing carbon (incomplete burn). If you replaced the plugs and the problem still persists, you'll probably have to replace the ignition coil.

You can check that with a multimeter and some reference on what the impedance should be, but replacing it should just be as easy as pulling some wires, replacing the coil and re-connecting the spark plug and battery leads to it.

tl;dr: It's probably your ignition coil.