Stepper2.pde has been well tested by now. I don't see anything obvious wrong with your code. Overshoot? 2058? Those 2 words don't match. If you need 2058 steps, that's more than 2048, then you would undershoot with 2048 steps. That implies you are missing steps. As if you were going too fast, which you are not. When you hold the shaft it slips. But you are not restricting it right? I'm going to say something obvious here: Something is wrong! Hardware or software. Try swapping 9,10 in the code just to see what would happen. It only takes a second. Here's how I would troubleshoot. Go really slow, maybe 1 step/sec. See if it hesitates or moves with even time intervals always in the same direction?
You can get nearly double the torque with 12v instead of 5v, at the expense of extra heat when running fast.
See code...