Start with the keel
1. Is it in line (an offset of 1 or 2 deg. can make all the difference) also is the keel symmetrical (the same on both sides)
2, Check the mast is on the centerline and is not canted to port or stb'd... the rake of the mast can be adjusted to allow the vessel to tack quicker or get closer to the wind.
3, Gripping is a sign that the vessel is out of balance, the usual problem is the the ships head is forced down by the wind and the rudder is partly lifted out of the water (It's a common problem on modern yachts (And square riggers ) modern yachts tend to be much finer forward than the older craft which were much broader in the shoulders and had more lift (( A, put some ballast down aft ,B make a larger rudder ,C try moving your keel slightly aft or adding an extention on the after end of the keel...any or all of these suggestions should help.
4, As a rule of thumb, your main sail when it is hauled in hard should not be midships but just over the quarter (6 or 8 deg. off the centre line.)..hauling it in any more only heels the vessel over ..slows it down and can lead to a capsize... the jib should be hauled in slightly less than the main on a fore and aft rig
Try these and you should get some improvment
JIMMY
De Freebooter