Are you absolutely you have the correct headset for your headtube? I ask because you said you go the headset with the fork. The headset specs/type are based on the head tube, not the fork. Maybe let us know what the frame and the headset (i.e. p/n) are.

To me there can be only three reasons for what's happening to you: not enough gap between top cap and upper spacer or stem (you said that wasn't it); the expansion plug is slipping inside the steer tube (my primary guess); or there's some sort of bearing mis match.