I'd take a guess that it's either a loose prism, as Zorkikat suggests or a loose diopter-correction lens. How bad a problem is it? You could take the top cover off without too much trouble and have a look to see what's causing it....
Zorki: 1e (x2), 2C, 3M, 4, 4K, 5, 6
FED: NKVD, 1g, 2e, 3a, 3b, 4L (x2), 5B, 5C
Kiev 4, 4A
Yashica Minister III
Zenith C, Zenit C, Zenit E
Minolta XG M, Z1
"The way to see by faith is to shut the eye of reason."- Benjamin Franklin