crete, I used to get this when I was riding my bike. Tried various different seats, handlebar positions etc etc to no avail. Went to the docs, got sent for physio. Diagnosed as a spasm of the muscle overlying my first vertebrae or something similar (can't remember the exact term, it was a good few years ago now). Physio said it was brought on by my head/neck/shoulder position when cycling and to do all my exercises and not cycle for a few weeks. Tried this, and the first time I went cycling again, it came back. tried again, same happened. Anyway, after months of physio, no long term improvement, and no more cycling for me! Fortunately I can avoid cycling, but if you have a similar problem, it might not be so easy to rectify, tho the suggestions here are good in terms of ensuring you're seated in the best possible position etc. Good luck.