Carl Zeiss Vario-Sonnar 1.8/10-100 goes out of focus between 50-100

[Tibor Hencz]

I recently acquired a Carl Zeiss Vario-Sonnar 1.8/10-100 and it seems to go out of focus between 50 and comes back to focus at 100. It really feels like a mechanical step happening at 50 that moves an element somehow, so I'm not sure if it should work like this or not, but I think it should be parfocal, so this shouldn't happen. Any ideas?