Mine does the exact same thing.  I believe it is the old motor requiring
more current to initially start the motor than it was designed to .  I
flick it over on 2 or 3 and then back to 1.  I never run it on 3 for more
than a few seconds.  I believe this high speed is what fries the switch.
As long as your motor is quiet, I wouldn't worry about it.

                                              Jay  '85 Westy (The
Hindenburg)