Do you have a Cinelli bar in the stem?....because I thought that pre-1996 Cinelli bars are a little (26.4mm instead of 26??) bit bigger in diameter than other brands, so if you have a different brand bar in the stem, it might be too small for the stem to grip on it adequately. Or you might also have an older 26.4mm Cinelli stem combined with a newer 26mm Cinelli bar.

Chombi