Shockingly shocking anomaly with PAL Mega Drive 1 50/60Hz mod

[People?]

Sorry for the title, which might be sligtly of an exaggeration, I just absolutely  needed your attention.

I just added a simple 50/60Hz switch to a PAL Mega Drive 1 console.

Normally, of course, the thing runs at 50Hz. When I switch it over to 60 it works  - but sometimes at completely random, the screen flickers - once. It kind of almost rolls. If you have a 50/60Hz switch on a MD or SNES or whatever, you'll know that flickery thing that happens when you flick it when the system is running - this is exactly what happens.

Some clarifications:

It does never happen in 50Hz mode

The flicker lasts for about a second, maybe slightly longer - NOTE - the sound skips during the flicker

When the flicker has happened, the video mode is still 60Hz

It happens with both a composite video cable and an rgb cable

The problem does not lie in the switch itself, I've tried bypassing it, and there is no difference

I tried lifting pin 46 on the SEGA 315-5313A-01 chip, because it was directly connected to the 60Hz jumper, this garnered a permanent 60Hz mode, but the flickering still occurs

You might be familiar with the fact that these things can happen on many consoles when there is a lot of white in the picture, but this is not when it happens  -  it is entirely random

Sometimes it will take about a minute until it happens, sometimes it will happen many times within just a few seconds  - again, entirely random

Any ideas what might be causing this? It boggles my mind! I just cannot figure out what the problem might be. I have tried re-soldering everything but of course that didn't do any difference. I also tried adding a 2.2kohm resistor between the +5V line and the switch position (the +5V position is 60Hz). This did not change anything.

[albino_vulpix]

Does the same thing happen on different TVs?

[Tiido Priimägi]

I had to resolder the VDP to fix this flicker problem