My DS1 is normally pretty stable in a variety of wind conditions.
Do you have an actual “DaySailer” or a generic daysailer ?
I would check the rigging for starters. Make sure the mast is centered and perfectly upright in relation to your boat. I have heard of a similar situation and the mast was tuned tighter to port than starboard and aided the boat to slowly go over to port.
Also make sure your center board is completely deployed and not hanging up half way down.
Another thing to check is the amount of rigging (ropes ) you have aloft. Are any of the ropes a larger diameter than needed ? They can add significant weight and windage high up on the mast and cause an unstable condition. Bigger is not better when it comes to ropes running aloft.
Check this guide and compare it to what you are currently running for rope.
http://forum.daysailer.org/tech_rigmeas.php
Another tid bit about your mast. Is there flotation foam in the top of your mast ? You may have a mast full of old soggy foam that may cause it to be top heavy. Brad