The 737-300 has a form of autotrim called speed trim which cuts in during certain conditions (low weight, aft CG, N1 above 60%, flaps not up (for the 300), AP not engaged, sensing a trim requirement etc).
I hate it because it interferes with my hand flying and usualy overtrims. And when a bloodey stab trim starts moving around of its own free will like that, theres always that tiny doubt as to wether it just might be a stab runaway this time.