hi jrock
i agree with jscerbo it sounds like your dropping your right sholder over the top and not moving your weight to your left foot. sometimes its your feet that cause this, check your not comming up onto the ball of your left foot when you swing back fully. as this sway you back and makes you drop your right shoulder as you start downswing. try and keep weight on inside of left foot as you swing back, also feel like you move weight to left foot as you start to swing down as jscerbe said if weight on back foot you lead with arms and hey pass your hips and club dig into grass behind ball. start with a few swing slow, 70% power and work up i am sure when you swig fast you feel your dropping and that its your left foot that comes up to far in your attempt to get a full turn.
let me know how you get on
bill