One other thing to look at is balance - if you scope is out of balance it will become hard to drive the scope in RA, especially if you are increasing the frequency of the steps as this provides less time to energise the coils in the stepper.
One thing to try is modifying the drive / code for a 4 step motor.
Found a pdf that explains the theory behind increasing speed and the drop off of tourque http://www.national.com/an/AN/AN-828.pdf
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