Roofy said it's been *happening* (plural) every since he switched (singular) to the uk store. This implies that he switched over once, and now iTunes / iMatch is deactivated when he restarts his computer.
This happens to me as well, although it's been happening on most of my machines every since I can remember. I don't know why, but it's pretty annoying so I hope someone finds out what causes it.
I always have to sign in again and/or reactivate itunes match in order to access / listen to my music, which decreases the usefulness of iTunes match. There's been a few other threads about this but no working solutions that I've found yet.