Alco RS 3 Running Issues

[RLG53]

I have had an RS-3 for just over a year, bought it new in December 2021.  It's been sitting idle for about 3 months.  When I fired it up yesterday morming, it wouldn't move.  Lights came on, but no movement.  In fact, as I increased the throttle speed, the lights dimmed; the faster the throttle speed, the dimmer the lights.  I disassembled the loco, cleaned all contacts, and cleaned all the gunk from the gearboxes.  The decoder pulls power from the body via the two screws that hold in place.  Track power is getting to the decoder, I measured it at just over 12 volts AC across these 2 screws.  The decoder is hard wired to the motor.  I suspect either a decoder issue, or a motor issue.  I have not desoldered the wires from the board to the motor, so I have not isolated the issue.  Yesterday, I had the loco running good, but after about 10 feet, it slowed down and died.  Your thoughts?