Note: groups.io will be down for maintenance this evening, starting at 11pm Pacific Time (6am Tuesday 8/3/2020 UTC), for up to two hours.
Thanks for your help.
Today, I was back on my workbench with three new ATMEGAs ordered last week.
With the new chips, the frequency is now at 7,030 MHz.
I think that the old ATMEGA was somehow wrong.
Now all works fine.
Thanks again and best 73