Screen flickering and artifacting

Hi,

I just received a used wii console which was said to be in working condition.
However, I got some strange artifacts & flickering when starting the console as shown in the following video: https://www.youtube.com/watch?v=8XcEOrpe...

The Wii outputs to A/V which is then plugged to an upscaler and then to my screen via HDMI. I already tried with 2 different upscalers and swapped out the video cable but no luck. I disassembled the console to see if there were any sign of visible damage on the board but nothing apparent.

Does anyone have any hint about what could be the cause of the problem ? Thanks guys.