Ah, so it's only the 03 model or there abouts that does it?
I've got a slightly odd tube on my race bike, it's got some extruded lines along the length that slot into the grip, presumably to help hold it in place, but it's not too great as the grip does seem to slip a bit
.
Anyway, I had a 99-02 throttle grip lying around, tried that but it actually made it worse!