I doubt the processor itself is the source of this problem check the clock, and power.

Keep in mind that the NOP has to be "executed" just like any other opcode.


I also tried with 10kHz clock and it didn't work. I tried with another power supply and also with battery (pure DC).
Now it works with 24MHz crystal and cheap AC-DC adaptor without additional filter capacitor.
I really don't know why this NOP solves the problem.