Hi, thank You for the concern and answers.
The code is quite big (definitely should be reduced to concentrate on the strange behaviour only) and before disclosing needs proper cleaning and commenting. Later I will do it to find out the reason.
Now I want to concentrate on another task: adding a closed motor speed control loop.
To avoid this problem, better call lcd.clear(); only once at the beginning of the loop, when flag "dirty" is set high and then manipulate with the flag only. That works.
Wal