I don't petend, in any way, to have found a magical solution, but I can honestly say that, since modifying my hand postion many years ago, I have never suffered from fatigue/cramp when writing for long periods and I remember that there was an immediate improvement in the quality of my writing.
This is much simpler to explain in photo form, hence the two illoustrations.

The first is a fairly common pen hold with the thumb, forefinger and middle finger gripping the pen which is held at about 45 degrees to the paper.
If the thumb, forefinger and middle finger are all extended to an extent, as in the second photo, this automatically has three effects :-
1) The strength of grip is lessened which eases tension, improves control and eliminates fatigue.
2) The pen to paper angle is lowered, which can help ink flow (and enables upstrokes more easily with a flexible nib).
3) There is naturally less finger movement and more writing with the hand, which can produce smoother results.
If your hand position is similar to (1) give it a try. It is easy to do and, in my experience, can have immediate, beneficial results.
caliken