Random,
Sometimes, the current tone of the muscles can actually get in the way of finer control. Consider a body builder starting ballroom dance. The muscles may be in great shape, but not have the finer control dance requires. Writing can be similar. It is a much finer grained activity than moving or lifting things and uses muscles in a different way.
If you a using lined paper, which I do, then the relationship of nib width to line spacing also makes a difference in text quality. If I try to write within a college rule line, even with a fine nib, it does not look very nice at all. It looks much nicer if I use two lines for a character height and then write very slowly. Right now, I am more at a painting speed than a writing speed and have to very deliberately form each character. Even though I have been doing fairly finely controlled hand/arm activity for many, many years (computer programmer, mouse, keyboard, PDA stylus, etc.), I still have trouble when I try to make the fountain pen control finer than I am ready for.
On
James Pickering's Basics page there is an example of a 'double lined' writing device, two pens taped together. Perhaps practicing with very large letters may help to establish finer control and identify movements which are causing problems. If that works, gradually work towards a smaller text size. Another tool that may help with this is the
Lined Paper PDF Generator. You can generate wider lines to start with and gradually work towards narrower lines. Or, as the image on James' page, you can just use a whole lot of lines for a character height. More lines may actually be better because it will have positions for the top, bottom, middle, ascenders, descenders, etc.