I'm not sure, but it sounds like the engines are having trouble maintaining a constant speed because of the spool time.  The engine powers up to reach a speed, but can't power down fast enough and blows right through the speed and then has to power down even farther so as to slow, then blows through the speed again in the opposite direction and so on.

The only thing I can think of is to not use autothrottle and just finesse the power yourself.  That should give you a little more control.