How do I accelerate a stepper motor?
The basic technique is as you have tried to program. The time between stepper pulses if shortened to cause acceleration and lengthened to slow down. A real solution typically uses an "S" curve type acceleration profile that starts out with a slow ramp up to the acceleration rate and then as it nears the target rotational speed it slows the acceleration rate back down. 

In your case you may be getting to the point where you have increased the acceleration rate too much and the motor cannot respond to it.