I wonder if it's not the keyed part of the switch that is the problem, it could be the switch on the lower end of the ignition barrel, it sounds similar to the problem I had on mine but the other way round, i.e, mine would sometimes not turn on until I'd turned it on and off quite a few times gradually getting worse,
It's a common fault it seems and after buying a new switch (around 80 quid ) I never had another problem.
I'm sure if it can stick in the off position it could also stick in the on position, though you do say the engine turns off so I might be talking bollocks,
ur right and not talking boll...
it does not alway turn off http://www.advrider.com/forums/showthread.php?t=807417