An EPROM usually holds data for something between 12 to 35 years... So, a bit flipping over so many years seems like reasonable 
Would never thought about it though, great catch 
I know an eprom can loose data over many years, but that is not what happened here 
There is nothing wrong with my real MSX, but with the data I read many years ago from it's bios chip...
Now that I read it again, I discovered a different bit in the first file.
So reading out the original eprom went wrong a few years ago.
Capacitors are needed indeed, not in the schematic...
Just add a 100nF close to the 74LS670, and one close to the 29F010 (or 27C010) chip, and a 100uF/16V over the 5V.
Didn't make a PCB design for it, just made a quick and dirty wire PCB and added this to a 16k cartridge PCB I got from you roadfighter ;-).
After this worked, I scaled it down to fit into these beautiful injection molded cases I bought a few years ago (no idea where I got them from, too long ago...)
Looks good right ?