Hi Rixa
Most annoying problem you're having there. Mine's working v.well (when it's not scarring the poo out of me!)
RF shield:
Speaking for myself I've never seen the point - remove the whole upper half: More room, fewer cut fingers.
Image quality:
Like you to me it sounds like 'ringing' of the analogue signal somewhere along the cables...
...If my Indi's ribbon is causing no issues to me & your VGA cable has given no trouble in the past I wonder if it's possible you're just 'plain unlucky' & you've got a combination of lengths that is promoting the reflecting of signals - cable length = exact multiple of signal wave-length or some-such...
...maybe a different length VGA cable will do it..?
I suppose you could try wrapping the ribbon-cable with a grounded sheath or put one of those 'ferrite doughnuts round it if it does prove to be interference.
Regards.
*Edit*
Another thought:
What resolution(s) is your Amiga running & what kind of monitor are you using?
Could it be your monitor is making a hash of scaling the Amiga's output..?
For instance, if you have a TFT does the situation improve if you use a screen mode nearer the native aspect-ratio of said TFT?