Curling ganache! Help?

Having made a ganache I pour it into a frame (37.5cm x 37.5cm x 1 cm /
15" x 15" x 2/5ths"). The frame sits on an acetate sheet, which in
turn sits on a stainless steel tray. I then leave the ganache to
crystalize for 24-36 hours in an air conditioned room at 17 degrees
Celcius (62.5 degrees F).

24-36 hours later, I remove the frames and coat the top of the ganache
with a thin coat of tempered couverture. Once this has set, the sheet
of ganache is cut with a guillotine into 256 squares, each just under
1" x 1".

My problem: the sheets of ganache are curling up at the edges during
the crystalization period. Some of them are spliting into two or more
framgents. Then the problem gets worse once the sheets are coated with
tempered couverture. Any thoughts as to what is going wrong?

I used to get this with the occasional sheet, now I seem to be plagued
by it. For information (should it be relevant): The current relative
humidity is just over 50%; my ganaches are made with approximately 9
parts cream to 10 parts couverture. This seemingly 'wet' ganache ratio
is partly for reasons of taste, and partly a peculiarity of using
Valrhona's grand cru couvertures - a 1:2 ganache resembles a brick in
texture - honest!

Many thanks
Simon