Grob Twin Astir Tailshaking

On a recent thermalling flight with a friend in the front seat I commenced a
high speed run from 3000 feet to lose height and demonstrate glider flight at
higher speed. I trimmed out for 100kts and flew a straight course until down
to about 2000ft where I then turned left 90 degrees. This took the
glider beneath some late afternoon cumulus that was still providing lift.
Immediately the glider began fishtailing rapidly giving the impression the
horizontal tail surface was moving side to side by a few inches. It sure got
my attention and I immediately opened the airbrakes and raised the nose to
reduce speed to about 60 kts. I honestly felt there was some chance of
structural failure.It gave another shudder and then settled down. I asked the
previous owners of the glider, another gliding club, if they had experienced
this phenomenon but nobody had. However another of our instructors had a
similar experience back in February and again late last year. The engineer is
not too concerned as he has not found anything loose or obviously amiss that
might cause this tail oscillation. It does seem to be rough air related as I
have carried out loops in this glider where I have dived to 105 knots with
none of the tail shaking evident. Our glider is the fixed gear G103 Twin
Astir.

I wonder if anyone else has experienced this. I cant find ant relevant service
bulletins that address the matter.