Arduino Motor Shield for 12v stepper motor
If the motor is rated for 12V and 1.7A, then supply it with this. USB can supply 5V at a maximum of 500mA, so instead of a maximum of 12V * 1.7A = 20.4W you have a maximum of 5V * 500mA = 2.5W. That's big difference.
My guess is that due to the DSLR's inertia and the weak stepper coil drive, it it slipping occasionally (or something similar)
Also, from a previous question I recall the Arduino motor shield is not the best design - the L298 drops a volt or two through both it's source and sink drive transistors so you will end up with even less power available, maybe around a watt (see source/sink saturation voltage in the datasheet spec table)    

The basic code looks okay, but it's impossible to say for certain as all the real functionality is tucked away in the stepper library. I'll assume this is probably okay if it's a commonly used library that's been around for some time.  

So, yes, supply the motor with the 12V and at least 1.7A. If you have to cut the trace then do this, it easy to solder it back later if you need to (make sure there is no jumper on the motor board first though, I have a feeling I remember a jumper for the purpose of an external supply on one of these shields)