I’m back, after some fiddling I realized that without any delay it doesn’t work. Instead I removed the PAUSE and replaced it with a loop. Now, this part of the code looks like this:

for a = 0 to 2000 ‘ ten turns
ST = 1 ‘ start pulse
for b=0 to 59:next b
ST = 0 ‘ end pulse
next a ‘ loop

This runs the motor slightly faster, however if b < than 59 the motor loses all torque and just hums. At about b = 65 or higher the torque is acceptable and I really do not understand why it can't run faster.

UCN5804B: http://www.taomc.com/bits2bots/5804.pdf

Thanks!