I have desuldered the PPU and tested it without any issue.
On my install I'm using the RVB + Sync pins on the AV OUT that are linked to the RVB + V (for Sync) on the NESRGB

I have also cut the trace going to the original pins. Everything is tested on multimeter and all contacts seem good.
The results is that when I turn on the nes I usually get the fist image of my game and then the sound get stuck on a note and the image start to break (in less than a second).
I can clearly see the first image of the game but just for a second. I can't start playing game so it doesn't seem like an image only issue.
I'm clearly complicating my life trying to do this on an original AV Out but I know it's possible.
Here's an image of the board if you're wondering. Wires are attached below with Dupont Pins

Any help would be appreciated.