I supposed it's not normal that assembler are "jumping" while they work ? feel realy weird

Jumping? are the machines physically moving around, or is it just the animation is pulsing?

No they stay in place but yes the animation is pulsing.

Sorry about the delay, you should look at some of the Deadlock stacking mods with the additional deadlock stacking recipes mod, the problem is that assembly machines have limits. You could also be reaching input limits, where you just cannot feed it fast enough.

I started a new game since few weeks and it doesn't seems to do it this time, so i don't know what was the issue, and i use Ultimate Belt + Ultra fast Inserters so feeding the assemblers is definitely not a problem.