stepper motor driver speed problem , PLZ read one time

[sizala]

guyz i am from india, i stuck at a point . i assembled a l297 and l298 driver, the driver is working nice but the problem is = the stepper motor is not running more then 150 rpm. if i try to run the motor more then 150rpm it just vibrates.

i dont know what is wrong.
my motor is 20KC/CM (or 280oz in)6 wire unipolar (i am using it as 4 wire bipolar) 1.5 amp per phase.
I am using 2 X 12V 20AH = 24v20ah battery as a power supply.

i am running that motor at 24V 2amp.
i am attaching the driver file and images...
plz plz help...

(i seen on google videos that other guyz are getting 1500-3000 rpm from the same driver )