1) Try to begin the descent with elbows leading forward and not to the sides.
2) Descend a bit slower so the bar doesn't bounce on your chest when it touches.
3) Rest a second more between each rep so you can get tight and take a big breath.
4) Try and improve the spotting situation: I realize this may be hard, but try and move the rack away from the wall so the spotter can be behind you and stays out of the way during your reps unless you actually need help.