Update:
I've opened the console and cleaned all contacts using qtips and isopropyl alcohol. Unfortunately I couldn't take the metal shielding off as it doesn't have screws, it comes with little metal bent lips keeping it into position. How do you take the metal shielding off?
After having waited for the alcohol to dry up I fired the console and to my surprise 4 out of 13 games worked straight away.
The remaining games always show a dark gray screen or occasionally some vertical colored lines.
Oddily one game, solaris, only works after loading it after one of the working cartridges. If loaded after one of the non working games it exhibits the same dark gray screen.
So load a working game (i.e. space invaders)
load solaris (works)
load a non-working game (i.e. moon patrol)
load solaris (doesn't work)
Lastly wiggling the video cable creates strong interference so the cable has a sweet spot which makes games look better. Do you think are my cartridges the problem, the atari2600, the video cable/connector?
What are your thoughts?
Thanks