The reason you lose so much weight when you stop taking it is because the majority of weight you put on while using it is water weight. Creatine makes your muscles absorb a bit more water, which can make you look slightly bigger, and certainly adds pounds because you're not peeing the water away. Studies have frequently found its effects to be very limited, unfortunately the studies I know about I have in print, so I can't really give a link
Another option is Biotest's
Surge. It's expensive, but I've found it has had much more consistent results.
As far as the plateau on the bench goes, I'd personally recommend not limiting range of motion. Having unbalanced strength in a muscle as it goes through its range of motion can lead to problems, often in the joints, later on because of inbalance. I'd recommend moving away from the flat bench for a bit. Try incline and decline if you haven't already. Try to do some chest exersizes you haven't done before, or in a long time. If you stick to flat bench your body gets bored of it and doesn't want to progress any further. You can do other free weight excersizes (dumbbell fly etc.), cable crossovers, classic pec deck, and try incline/decline lifting with dumbbells instead of a bar. Another you can do is vary the instensity/speed of your workout. Try supersetting for a week or two, if you've been doing the same old number of reps and amount of rest repeatedly.
Hope that helps.