Megadrive graphics glitches after installing 50/60hz switch?



I've installed a 50/60hz switch on two PAL Megadrives, and both cause the same graphics glitch in both 50 and 60hz mode.

In both mods the cutting of the jumper trace failed at first, causing a short circuit (and switching off of the console) after flicking the switch. a deeper cut of the trace fixed this.

Afterwards though, there were slight graphics glitches to be seen in all sonic games and, for example, rocket knight adventures. These glitches appear only on the water effects found in those games. Where the 'air' meets the 'water surface' in levels like the labyrinth zone in sonic 1 or the horizontal flying level in Rocket Knight, there's a line of weird flickering pixels to be seen exactly on the line that separates the water from the air. It seems, therefore, to have something to do with transparent water effects and the like. On the reflecting lava level in Rocket Knight there's the same glitch.

Does anyone know if this could have something to do with the shortcircuiting of the 50 and 60 hz jumpers? And why did it have the exact same effect on both megadrives, and only on certain graphical effects in different games?

PS: I originally posted this under:

...where a couple of replies were already posted. Wrong section, my mistake (I'm new here)...