Hello, it is normal for the motor's behaviour to change when the pot is touched - that's why lots of the drivers come with a special ceramic schraubendreher for adjusting them.
What bit of Polargraph code are you using?
I can't really see anything wrong with your circuit diagram. It is not a polargraph circuit (no enable pin connected), but that should not prevent it from working - only from turning off. I don't think that the vibrating is normal, but it could simply indicate a floating "step" pin, that isn't connected, or that hasn't been initialised and is waving about, randomly triggering steps.
You will probably have modified the source code (which version? from where?) to have something like:
const byte motoraStepPin = 30;
const byte motoraDirPin = 32;
const byte motorbStepPin = 36;
const byte motorbDirPin = 38;
If you think there is something wrong in the physical circuit, I'm afraid a diagram will not help. Only a good, clear photograph will.
(Secondly, you have resurrected a 24 month old thread - it would be better to create a new thread - I'll leave it up to you to do that in future - as I'm not clear if you have used lanthan's firmware or not.)