porpoising is when the nose of the ski tends to "porpoise" up and down like your rolling over speed bumps.
generally its solved by speeding up/slowing down, trimming down or by getting off the center of the hull by making slow turn left or right, only to start porpoising again when you start going in a straight line and the cycle repeats, over and over all day. The yamaha GP hulls are probably the most famous for this, and the X4's aren't much better.
I never really noticed it much on the GSX but I rode an xp for quite some time so maybe I was just used to it and didn't pay attention.
As far as stability, the GSX is pretty stable IMO, certainly not as bad as other models, but not nearly as good as the new ski's which are quite a bit larger/wider.
If your new to the sport I recommend just spending more time on it and becoming more comfortable with the balance needs.
Are you usually riding solo or with a passenger ? Riding with a passenger can certainly impact the "tippi-ness" factor.