Cannondale Aficionados / Owners

I dunno. The stem should be doing the work of keeping the preload. You can ride a-heads without a top cap or star nut or bung or whatever just as long as the stem is gripping the steerer properly and the correct pre-load was applied and locked in by the stem before the top cap was removed.

I'd wonder if the compression ring is too low. I'd try running it without the headset top-seal doobry using the stem to directly apply pressure to the compression ring. See if that solves it.