Ed,
   I had a problem with my S-Tec 55 doing exactly what you described not
long after I started flying my ES.  In my case, the cause turned out to be
slack cable tension between my pitch servo and elevator pushrod.  I could
feel the play in the pitch axis by engaging my A/P on the ground and finding
that I could move my stick slightly back and forth rather than it being
tight as it should have been.  The cable was tight when we installed it, so
I can only assume it relaxed a bit once I started flying.  I tightened the
turnbuckles to reestablish the proper tension and everything's been fine
ever since.
  Odd that yours would be OK for three years before exhibiting these
symptoms.
   Skip Slater
   N540ES