I'm making a simple puzzle-building game. It's working, but I have just spotted an odd in-game behaviour...

When I pick up a piece of the puzzle and move to the left side of the swf, the game runs at a playable frame rate. The farther I move the mouse to the right, the frame rate (or at least update rate) seems to drop.
The only code it's running at that point is a method that checks which piece is carried and moves that piece's x and y coords to match the mouse.
I've already tried using 'updateAfterEvent' with no change.

Has anyone experienced something similar, and if so do you know what the problem is?
Cheers.