Shoulder instability on bench press

[Gun Nerd]

I'm looking for some help with my long-suffering left shoulder during the bench press. I have a terrible time locking the scapula on that side in place. My left trap tends to pull it upward while I'm lowering the bar, resulting in possible impingement (at worst) at the bottom, and pec/delt/bicep strains (at best) on the way up.

I really don't have the same problem with the press; gravity helps.

I'd greatly appreciate suggestions either for bench technique cues, or for assistance exercises that might help (and that don't involve half an hour twice a day with 2 lb. dumbbells and rubber bands).

[Steve Hill]

Video of the unusual phenomenon?