Something's not right if it's moving on its own...Having said that I hold the throttle in all but the most benign conditions!(over 95% of the time at a guess)....When its ROUGH or even Choppy I never take my hand off the Throttle Base (as said earlier) at all...in the "Heat of Battle" you are constanly changing speed/revs to cope with and pick your through what's in front of you...I definatly couldn't let go of the Throttle and loose that control even if I wanted to!!
The friction plate is easily accessed and adjusted as you know through the rubber grommet/cap on the side of the Throttle Handle....it might pay to disassemble and have a look see...Maybe it's be over lubricated or worn
either way I'm sure it's an easy enough fix..