I have got it sorted nothing to do with ram.
The example in config.txt is config_hdmi_boost=4
Which stupidily I took for the highest safe value.
In the above “You may experiment with different values of config_hdmi_boost. The default is 2 for Pi1 model A/B, and 5 for newer Pi’s (B+, Pi2, Pi3). At your own risk, you can go up to 11.”
Made me assume it was similar to overclocking the HDMI vcore or something but apparently its a EMF problem more than anything where you can run at 11 safely just migt be throwing out interference.
I have config.txt config_hdmi_boost=7 with just the odd green pixel shimmer.
It good enough and actually hard to notice.
Still don’t understand how the same cable with a passive DVI convertor works and the same HDMI cable straight doesn’t as the signal difference must be tiny, maybe slightly worn conacts… ? Just dunno :)