Well it's hard to tell if it's the ROM chip or the Mega II. I also played around with the scope and followed the schematics, but I'm not able to pinpoint the root cause.

Here they are if you are curious

Schematic Diagram of the Apple IIgs.pdf

I can desolder the Mega II if I find another one.

I'll try look further...

Thanks for your help and suggestions!

Pitou!