Hello.
From your description, it sounds like you might be reaching the step rate limit of your current setup. There are a lot of factors that go into how quickly you can step a stepper motor, including operating voltage, current limit, microstepping mode, acceleration ramping, the load on the motor shaft, and more. What do you have the VREF voltage on the A4988 carrier set to?
I do not see any datasheet in the listing you linked to, but if you can find one for your motor, they commonly have a pull out torque curve you can use to get an idea of what kind of step rates are achievable. The listing you linked to does mention 50 RPM, and at 1000 steps per second, in full-step mode, you are well beyond that at 300 RPM. You can find some general suggestions for getting a higher step rate in this forum post by Ben.
Brandon