Sorry Stig, Had to do it...
Did you see my post on testing the O2 sensor?
You can pull it from the pipe, and using a DVM, and a propane torch you can see if it is reading.
I don't believe you have a sensor problem tho. It would have shown on the ECU UNLESS it is a bad ECU.
The O2 sensor will create approx 1 volt after 2-3 minutes of heating. Remove the heat, and it should drop back to 0 volts in 2-3 seconds. A second test it to disconnect the pigtail, and run a wire up to one hand from the O2 wire that goes to the ECU. Attach a wire to the positive battery post, and your other hand. I know the bike has to be under load to see the problem... Bear w/ me. Ride the bike w/ these wires in each hand, and see if the problem clears.
The idea of this is your body acts a a resistor, and it will allow approx 1 volt at some milli amps to trick the ECU into thinking the O2 sensor is reading.
Reaching yes but it's better than fort'ing up.