PX245 / PK245 step motor jerking problem

So I am replacing the very slow 64 times geared down 28BYJ48 step motor by no gear PX245.  But there is another problem - the motor is jerking a bit.

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def Test4StepMotors(stepSequence, stepAngle, stepTime, direction, degrees):

    ftprint.PrintDoubleSpaceLine("*** Start testing stepping motor ***")  

    print "Step sequence = ", stepSequence
    print "Step angle = ", stepAngle
    print "Step time = ", stepTime
    print "Direction = ", direction
    print "Degrees = ", degrees

    spiChannel = spidev.SpiDev()
    spiChannel.open(0, 0)

    stepCount = int(degrees / stepAngle)

    if (direction == Clockwise):
        for i in range(stepCount):
            for stepPattern in stepSequence:
       stepPattern1 = stepPattern
stepPattern2 = stepPattern << 4
stepPattern3 = stepPattern1 | stepPattern2
spiChannel.xfer2([stepPattern3, stepPattern3])
                time.sleep(stepTime)
    else:
        for i in range(stepCount):
            for stepPattern in reversed(stepSequence):
                stepPattern1 = stepPattern
stepPattern2 = stepPattern << 4
stepPattern3 = stepPattern1 | stepPattern2
                spiChannel.xfer2([stepPattern3, stepPattern3])
                time.sleep(stepTime)

    spiChannel.close()

    ftprint.PrintDoubleSpaceLine("*** Stop testing stepping motor ***")

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