You have not said what you are powering the motor with - volts and amps available?
My guess is that you need a driver that can handle a lot more current than an L988 can. I think I had a motor like that years ago that could take 9 amps for a short period. And for that I would use a driver that could handle about 15 amps.
Maybe an Electronic Speed Control (ESC) would be an economical option. They are controlled like a servo.
...R