Thanks again for your help. Maybe if i had read the entire datasheet i would have known about these evil capacitors !
You are not the only one making mistakes PaulRB : i asked for 0.1uF capacitors at the store, and they gave me 0.01uf capacitors! Fortunately i realised that... just after soldering all of the 43 capacitors.
But even with the wrong value, it works well almost always. I still need to run some more tests but if i notice occasional errors i will just replace them.
About the 74HC04, i use them in pairs. My debug might have been messed up by other errors, but i'm pretty sure these buffers apply a tiny delay, so i made the DATA wire go through it too so they stay in sync.