Random flurries won't work. You can't just rely on speed. The head should not be forward. Keep the head back or it will get hit and it also brings the whole posture down to stiffen up the hands. Keep the head up and rely on your touch.
Many students use too much force.
Wing Chun is a ladies' style and therefore brutal energy or brute strength should not be used. We are all getting older, some day you can no longer rely on your muscular strength. The speed and power approach is limited. You can only take that approach so far. Ken has yet to see a limit in the soft approach.