From PJ2's referenced post of Aug 2009
I watched, as both horizontal stabilizers began fluttering up and down describing an arc of about 1 ft or so. They were fluttering at about 6 to 8Hz for about 20 seconds gradually damping then beginning again after about 40 seconds. The pattern did not vary with Mach or altitude. It would gradually stop after some time in cruise.
The danger of treating this type of problem as "Ops Normal" is that it is using up the fatigue life of an important part of the aircraft. If people are conditioned to routinely accept this type of vibration, then someday, somewhere, someone will have an exciting experience as the next level of failure makes itself known.
Took several months to fix.
Is not acceptable.