I've had a couple locos that would periodically require me to manually spin the motor/flywheels to get it going again and I came to the (fairly certain) conclusion that both were being caused by a bad motor pole. When I was still just doing DC, these locos would run fine once going but, once stopped, they would sometimes not start again without a manual motor nudge. One telltale was the stall would always occur with the motor armatures in a specific position. I mentioned these locos ran fine once going again on DC, but having a bad motor pole seems to wreak havoc with a DCC unit (all kinds of strange speed fluctuations, etc.). All this said, making sure all contacts are clean as suggested above is a good first step.