I've used CRT's and 5:4 tft's 17" and 19" either multisync or with a scandoubler and results vary and also change through environmental impact/interference. Your A1200 video out (if it's RGB) probably has a stronger current now with new caps so you could explain the picture that way. I'm using a 5:4 19" NEC accusync atm and I found the best way to center the frame is to use a corresponding screen-mode - in this case Multisync 640x4xx and set the Overscan settings in Prefs. Any screen will have a better time with sizes that compute!
