knut_ny:
radio interference is not the problem.
check for memory leaks. (may use monitoring software for this) og good decoupling on power supply lines
Now I'm more confused than I already was before; another person (not on this forum) who was an antenna himself in a previous life (if you'd ask me 
claimed that it most likely would be high frequency interference. I'm a math guy and I claim utter incompetence in this field ...
b.t.w. I am quite sure that the issue is not a memory leak or a bug in my code; it is more than two years old and I scrutinized it again this morning and above all, it runs flawlessly on my old AtMega2560.
Thanks for replying and
kind regards,
Jos