Is there an error number?
Short answer without a peck or copeck report no telling.
it is a 300 - however the SP went from SP300 to SP300v to SP300 so which one is yours - all are old but the hours on the motor can determine if it is at its end of life.
Then you have the encoder wheel or its sensor that could be having issues.
SP 300v: Error code 0008 0008. Machine will rewind after printing for drying purposes and rewind enough to begin next print job. Just will not rewind if print job is longer and has to rewind to start cutting. I may not have enough heat in the area where the machine is. Could that be my issue?
....are you using a take-up?