Most AC difficulties, that aren't injuries, are due to strength imbalances in the shoulder. Rotator cuff work should do the trick. That and a targeted exercise for the pectoralis minor to help stabilize the shoulder. I would probably suggest seated external rotation, Cuban rotations, and the scapula depression.
This just killed me last night. I don't know if it was the added ice, but it felt like I'd been hit by a baseball bat.
I'll have to quit DB Pressing for a few weeks. Hopefully I can still row and deadlift, though those put a lot of strain on the old shoulder girdle as well.