Isn't your bullet a little larger than it needs to be for the bore of that gun? I use either a .451 or .452 and get excellent results.
Each gun can be a law unto itself. Even identical guns from the same manufacturer can vary some in barrel dimensions. Without slugging the bore of the individual gun, you don't know exactly what the grooove diameter is, and what to size it to for best results. Most feel .001" over groove diameter is best all around.
I am using a Lee .452 mold. 90234. It was actually giving me larger bullets and bullets that were not very round....
It sounds like you're on to the molds quirks some, but having round bullets should help. It shouldnt take a clamp to keep the handles together, but if that's what it takes, then who am I to complain?
Do the handles touch when the mould is closed? If so,try clearancing the handles till they don't, it should help if they are touching now. I've seen a few that touched, no good can come of that. The blocks need to be tightly together, not the handles.