Modolo Q-Even Stem
 

I am looking for any help/advice with Modolo Q-Even stems. I've got a 1990 Cannondale SR900 that came with that stem. It's the original stem and bar combo. No matter how much adjusting I do or how tight the stem is, the black plastic insert on the stem is still loose. Is this normal? Am I being to OCD? It's not super loose but enough that road vibrations would probably cause movement. I've tried the original stem, and two others (one NOS). All the same results. The bar is definitely tight, but still that damn plastic piece moves! Any help would be appreciated. Thanks.