monitor goes standby during work (unwanted)

Knoppix works fine, but in regular intervals (6 minutes) the monitor goes on standby, though I am continuously working with the system (mouse moves, keyboard access). When I type a key, the system wakes up. Some keystrokes get lost when the system goes standby. It-s not a dramatic failure, but annoying

In Knoppix, Kontrollzentrum, Energiekontrolle, Energie I tried both settings: all functions disabled, and all enabled with 60 minutes. It does not change the behaviour.

(In Windows, power management works as expected - disabling monitor, HDDs after a certain period of inactivity).

Any idea?