I used to have an S2A and I have to agree with Mongo, everytime I cocked the shutter I squeezed my eyes and said a prayer... There's actually quite a lot going on with the S2A when you take a picture. First the mirror comes down, rather than up like most SLR's. That means the light comming through the focusing screen needs to be blocked out because the mirror isn't there to block it, so a curtain covers it. Then there's the shutter, which is a curtain. And I was also told another curtain covers the mirror to prevent light from bouncing off it... so there's quite a lot going on and I'm sure so many mechanical things can occur that might cause problems.
I wouldn't have guessed the lens itself would move... Does the camera work fine, and do your negatives come out ok? Is the lens locked on completely? How about the helical focusing mount, is it attatched to the body securely?
If the problem isn't in the focusing mount (in which case you can buy a new one) then unfortunately I wouldn't have a clue what to do... except take it to the doctor!