When I was a kid, 12-14 years old, my father who had a garage used to let me drive cars behaving as yours until I burnt away all the soot..., normally the top speed in the beginning of the session was 80 km/h and in the end almost twice.
In your case it is probably one or two bad spark plugs, but if they´ve been bad for long you also have got a soot layer in the cylinder head. Normally it is a combination of both causing the motor to continue to run some laps.
The reason behind all this, refering to my childhood, was a too rich mixture and sometimes too little reving...
If your plugs looks clean and fine, I am totally wrong...