If yer running some sort of padding between the engine mounts and the frame it may not be grounding itself - even if your not such a passive ground isn't to be trusted as there's paint and stuff so connecting both
the ground from the engine and the one frm the switch is the way to go (water bottle mount works well for the engine ground if ya have one).
Odds are you've just got a funky switch - they're pretty much junk. Although mine works well, many have complained of problems with them. If you've got a multimeter ya outa check that switch - if ya dont, make one w/a battery, a hunk of wire and a flashlight bulb.
If the switch is hinky sometimes you can fix them w/a lil surgery, or better yet check ratshack fer one that's about the same size (Radio Shack, "momentary on" switch) - or just put a toggle anywhere convenient...
or as michael mentioned - jus' choke it to death FTW