Hello again.
I was getting desperate
I had changed EVERYTHING except the "gearbox"; the transmission from the motor to the wheels. Could there be something wrong there? I could not see or feel anything wrong when I checked. But I removed the motor, and cleaned all the wheels very carefully, and re-oiled them (carefully, only 1 drop of oil). Still the same problem.
As a last resort, I added some very thin model-grease in stead of the oil. This is a little thicker than the oil.
NOW IT WORKS! It now runs very fine in both directions.
Strange....?
Best regards,
Per.