For most of us the swing mechanics stay somewhat consistent but it is the tempo that changes and causes our problems. If I play with a couple of my co-workers here, I end up swinging much faster (although I try not to watch them) than I would like and therefore my game suffers somewhat unless I am CONCENTRATING fully. So for most of us, the ups and downs come from maybe doing something different in our setup routine, gripping tighter or swinging too hard/too easy. Usually the swing stays ok. Swinging harder tends to force us "over-the-top", swinging too fast results in not finishing the backswing to a full coil and therefore we are playing catch-up all the way, swinging too easy or trying to steer it tends to result in weak-fades that go nowhere and the list goes on...mainly it is TEMPO and TIMING that change, not mechanics.