Thanks skimask!

Yes I did miss your point; of curse the "squarewave" must look better to the 5804 if the duty is more like 50/50!
New code:

for a = 0 to 2000 ‘ ten turns
ST = 1 ‘ start pulse
for b=0 to 28:next b ‘ on time
ST = 0 ‘ end pulse
for b=0 to 28:next b ‘ off time
next a ‘ loop

The motor runs at the same speed as before, perhaps with a little less noise. If I try to go below 28 it loses torque and I do not help to use different on and off times. I have tried both ways, shorter on time and shorter off time.
S**t, I really thought this was the solution!