Got this pcb to be repaired from a friend.
The pcb was working perfect excepts that the screen was flickering at 1/2 of the vsync.
As usual I visually inspected the board to find loose legs on smd chips but the only one present was good.
I noticed that the pcb had 4x rams 4164 near the video custom chip which are known to be not very reliable.
I started to probe the rams until I found one which had all the signals very weaks.
After soldering a new one, the problem was fixed!