Update : Day 3 and still all PIR nodes are working ;)
I´m using the given sketch here.

Still up and running stable.
8Mhz Bootloader did the trick for me :)

@siod
I removed both to make the smaller ( HC-SR505 ) work with ~3Volt and bridged these contacts as shown in the picture

They work perfectly fine for me and are not too sensitive.
Actually they dont even react on my cats, which is nice, because the "bigger" HC-SR501 always did... :)