GWX is much more memory intensive than the stock game and you may be a little light on RAM at 2gb, which will account for the jumping of the sea.
I had this problem on one of my machines when playing war ace campaign, and when I updated the RAM to 3gb it went away.
Ideally you need at least 2.4 GZ processor too, which it sounds like you have.
If you cant clear the fault by updating graphics card drivers etc, then RAM is cheap and easy to slot in for the benfit it will give you on this game.
