Probably of the items that I mentioned, the spasm is the most likely; I'll elaborate on why I think this is more than a remote possibility if you wish, but just for now I'll just mention that one very telling indicator is the sudden cessation of the symptom when just getting airborne for instance. Some of the data gathered, from an admittedly small sample of pilots, have indicated that muscular tensions, headaches and stomach pains, in some cases long term chronic symptoms, disappear completely during critical phases of flight. It seems that the level of concentration transcends any form of relaxation technique. I would guess that even if it is the inverse of this is true, and it gets much worse during these periods, it is all part of the same function and an indicator that you will be able to control it eventually.