Atari ST RGB won't sync unless an RF cable is plugged in at the same time

Setup - Atari 1040STe plugged in to my CRT TV with an RGB cable

The picture rolls/jumps but when I plug in an RF cable it stablizes and RGB works OK though not 100% perfect as in it is stable but very very slightly jelly like, i dont really know how to describe.

It doesnt make sense that it needs the RF because it will still be displaying RGB and set on AV1 on the TV.

Composite contains a sync signal, which appears maybe missing from the RGB cable. That would explain why it is stabilising when you plug in the RF.

i would be thinking there was a grounding issue...

Could be. If it's a SCART header, pop it open and have a look.
Check that Pin 20 is hooked up, that is where you would get a Sync signal from. Pins, 18 and 17 are GND and should be connected..
Pin 16 selects RGB, this should give you 3V when hooked up to the Atari so that the TV knows what signal to expect.
If you have anything different to that, let us know.
For the pinout, have a look HERE