Aha ok so in that case a 90 degree angle would be like 90/3.6 counting 25 jumps, if makes 25 jumps then seems right.
I see your motor is unipolar 6 wires, so have you tried using only half of coils instead of full coils. Although that probably wont help as motor resistance is already kind of low. Cant read the voltage on that supply but if its too low like 5v or so maybe current wont reach the correct values in due time, that could be fixed with an atx 12v psu supply. If it does that with 12v supply maybe the coil resistance is more than 1.2 ohms, you can double check that because custom motors dont follow datasheet, so different specifications for that motor its not out of the question.
I see your motor is unipolar 6 wires, so have you tried using only half of coils instead of full coils. Although that probably wont help as motor resistance is already kind of low. Cant read the voltage on that supply but if its too low like 5v or so maybe current wont reach the correct values in due time, that could be fixed with an atx 12v psu supply. If it does that with 12v supply maybe the coil resistance is more than 1.2 ohms, you can double check that because custom motors dont follow datasheet, so different specifications for that motor its not out of the question.