Very common, so don't get too frustrated.

The woods swing and the irons swing are similar except in a few ways:

WOODS:
1) They want to have the club sweep across the ball. Almost lifting it up in a "low to high" motion. Take the club back low to the ground for as long as you can using only the shoulder turn. Then let the rest of the swing take over. Try to swing up on the ball more and finish with you hands high in the air.

2) The hands need to relax more. Take them through their full motion, but don't try to use them as power or acceleration. Let them just go thought the motions. Get the power from the hip and shoulder turn.

If you start to do well in the irons and are having trouble with the woods again, my bet is that you are using the iron swing: taking it back high and abrupt, and trying to hit it hard with the hands.

Hope this helps.