Capt. Haddock wrote on Jan 19^th, 2011 at 10:18am:

This has happened to me every time I've manually adjusted the trim whilst the AP is engaged, so I guess it's just a feature of the aircraft; I just switch to altitude hold until the 'plane levels out then everything's OK and I can continue with the climb. >^..^<

Well, there you have the answer then:
You should never touch the trim with autopilot engaged...its fighting one against the other.
If autopilot is engaged, IT controls-adjusts trim.
I can imagine strange thing would happen if you touch the trim with AP engaged, (Ive never even tried it), nor thought someone would, so the bouncing could well be because of that.

Another example: Its like switching autopilot altitude hold, and then try and climb or descend..."why would you do that ???" its common sense, (no need for a pilot licence on this one) either its autopilot or manual, if its autopilot..DONT TOUCH stabilizers.