I have tried to disconnect knock sensors, the problem still exists.

Actually, hesitations and vibrations only appears between 1000 and 2000 rpm, on strong acceleration. Sometimes it also happens during small acceleration, and, very strange, sometimes, it doesn't appear at all.
The problem exist also when I don't drive, when I'm on neutral gear.

Where can it can from ?
I have a second engine for parts and I changed MAF sensor, injectors, fuel pressure regulator, spark plugs are new.
I will verify the timing belt and the "spark plug distributor" (don't remember the english word for this part)