Chesful wrote: Ok, I've found the problem.
I was using another 2N7000 in Q6 instead of the MPSA42. Obviously, I've surpassed the Drain-Source breakdown voltage with the neons and broke the transistor. That's why my neons were so bright and didn't "blink" (they where always with the same brightness, I've should have stated it more clearly in my first comment).
Also, I've should have checked before the datasheet of the component. Although it is said clearly in the construction manual, the schematics show that Q2, Q4 Q5 & Q6 are all 2N7000 and thus, my confusion.
Now that I've replaced it, it works perfectly.