It sounds like this might be an early (<1998) PS1. If you're using a chopped-wave power supply, intermittent or varied chuff rate is a known problem. You would have to use a sine-wave transformer, or put a 150uF NON-POLAR capacitor across the output of your transformer to smooth the waveform.
It is possible to adjust the chuff rate, but as I remember the procedure involves applying power while holding the loco back, etc. That's pretty hard on the motor and tires!!
