Flickery SNES output

[Vertigo]

My modded PAL SNES has a badly flickering video output.
It previously only affect my erm... 'third-party backup and development device' because that was drawing a lot more power from the cart slot than a normal cart, but now it's got really bad, I get an extremely flickery screen (like as if the sync is very slightly off but no rolling picture) when using cartridges too.
The effect is worse in 60Hz mode where the screen jumps about up and down and is unviewable as well as headache-making.
In 50Hz mode there's a nice white line that gradually rolls up the screen, then back on the bottom again.
Sometimes it'll start fine and then start flickering after a while, often the system will flicker straight away for a little bit (so it's not a heat problem) and then reset completely, then just reset over and over after a couple of seconds.
I'm thinking/hoping that it's just a minor component (capacitor somewhere maybe?) that needs replacing, something small, easy and obvious.
I want to fix it so I don't have to use the ugly US SNES I have for very long.
Anyone experienced this before and got any ideas?