xbee causing odd servo jittering in APM?

Here's an odd thing that happens in my office.  I haven't noticed it happening in the field.

• when powered up, the servos constantly jitter
• jittering seems to be going through the APM... note the period of time when APM is booting and jittering stops
• jittering associated with 900MHz xbee
• if the xbee is disconnected, no jittering
• if I ground the xbee by touching the antenna, jittering stops

I'm curious as to the cause of this; partly to see if there's a problem, fix, etc, but mostly to understand what's going on. The primary practical issue is that the jittering noise seemed to make people nervous as they were looking at the unit.

update:  It seems to be noise from the radio.  There are quite a few postings on this in the diydrones forum.  Applying a ferrite ring on the cable between APM and xbee seems to be the recommended fix.