SEGA Megadrive Mk1 Hz Problem

[Qjimbo]

Hi,
I'm having a problem with european Megadrive mk1's. Basically it appears the precision on the 50hz isn't good enough for my TV tuner card, so it flickers around, I didn't have this problem with my old NTSC Genesis, but that borke recently, nor did I have this problem with an Asian PAL-B Megadrive (which I accidently ruined wen trying to mod to 60hz by replacing the crystal...)

So far I have purchased 2 Megadrives, both have had this odd flickering problem (One was modded to 60hz, though my TV card can't do PAL-60, so that was useless...). Can anyone suggest ways of improving the precision of the frequency? It appears there are quite a few versions of the Megadrive around.