on my Gammill if the timing is off, it causes skipped stitches. sounds like you have more of a tension problem. Last time I had a tech out for maintenance he said that long armers don't replace their check springs nearly often enough and that can be a source of stitch quality.
I too check tension each quality by stitching out on a practice piece and use my Towa gauge for each bobbin---even with the same thread, sometimes the wind of the bobbin will be a little different--each bobbin seems to have it's own mind!