I would guess the problem can be traced to the fact that the centre of effort of your sails is far too forward compared to the under water profile of the hull and the constant swapping of course is because your boat is always trying to find its equilibrium but cannot maintain it.
Try adding to the area of the keel in front of its current leading edge and sail again.
Or sail with mainsail rigged only or if this is too much try a reduced forsail area and see if this helps.
All will point to the centre of effort of the sails being too far forward if she then sails happily.
Having a longer keel will make her sail straight but you my find it difficult to steer but it will point you in the right direction before the need to start hacking at the poor thing
Now the technical bit
A simple way of finding the centre of effort of the sails is to cut the shape of forsail and mainsail out of cardboard as one pattern and suspend it from a length of cotton.
By playing around with the suspension point until the pattern suspends horizontally will give you the centre of balance of the shape and thus the centre of effort of the sails.
This point is transferred to the sails on the boat and projected vertically to the water line.
Next make a profile pattern of the underwater profile of the hull including keel and rudder at the centre line (side view of hull on plans) from cardboard and balance it on a straight knife edge horizontally so that the balance line is perpendicular to the water line edge of the shape.
This gives you the centre of effort of the under water profile which is again transferred to the water line on the ship.
The sails combined centre of effort at the water line must lead the centre of effort of under water profile by a little way, something like 10% of the water line length including rudder is a good starting point.
With a bit of patience and a bit of fiddling you will get the boat to balance across a reasonable range of wind strengths.