I just tried to edit my first post and had a whole long message typed . Guess where it went? Anyway, I'm not too awake yet but here's what I'm thinking.
Brother machines don't normally jam up, they just don't, so something is going on. The fact that it's intermittent is why the repairmen haven't caught it. Check every part of the hook/race assembly for a scratch or burr, anything that would catch the thread. Make sure that you're loading the bobbin into the bobbin case the right way. The only other thing I can think of right now is maybe the upper tension check spring is not working properly, either worn out or not assembled right.
Cari