http://willswing.com/blogs/PilotBlogs/tabid/38/BlogID/5/Default.aspx

New gliders do not flutter at ‘V_d’ dive speed however older sails
will invariably begin to flutter at much lower speeds. Fluttering will sometimes
progress from a short-intermittent buzz at high speed to a debilitating and
persistent beat at much slower gliding speeds with as little as 50 hours of
airtime. Fortunately, you should be able to coax as much as 500 hundred
flutter-free hours from a sail with just a little preventative maintenance.
Curved tip gliders are more susceptible to flutter because there is more
unsupported sail area aft of the roach line (defined by a line from the end of
the leading edge to the trailing edge at the aft end of the root cord) so they
require somewhat more attention and tuning.

There are three steps to correcting tip flutter. (See the link above)