Welcome to the board....If you walk around the boat are there any areas where your rub rail is loose? I discovered on my 91 190 that the rub rail track screws were also used to hold the top edge of the hull to the gunnel/top deck. This might be the source of your flexing.
If your rail is loose then it is worth pulling the insert and tightening the screws all around the boat. You might discover that yours has loose rivets (I think rivets are the way it would have come from the factory) which will need to be drilled/punched out and replaced (bolt and locking nut would be ideal). Please note that putting the rub rail insert back in is a huge pain so give your self plenty of time and maybe get a helper.
Here are two threads that discuss rub rails. Hope this helps.
Current - 2008 X2
2nd - 1999 PS 205 Blue/White, LTR
1st - 1991 PS 190 Red, 351w & Powerslot
Last edited by hester; 07-28-2008 at 03:28 AM.