I often have this issue with the Triang/Triang Hornby X04 motors but then I probably have an above average encounter with this motor. I would take issue with Doug and say that these motors were some of the finest and most reliable ever produced by any model railway company!
They are very easy to work on and do offer reasonable slow running qualities when using a vintage/classic analogue Hammant and Morgan Duette controllers. Its the DCC control systems that require the use of 5 pole motors.
Try cleaning out the carbon that builds up within the armature gaps. You will need a fine needle to do this but be careful not to damage the motor windings.
Fit some crocodile clips to the brushes and see if the motor still stalls when you apply power. If it doesn't then the issue is between the motor brushes and the wheels and it could be a pick up issue as over time the insides of the wheels can get very dirty or tarnish and the motor requires 0.4 amps to get going. You may need to replace the pick ups and clean the wheels.
Check the brush carbons to make sure they are making a positive contact with the armature.
Make sure the motor bearings are lubricated.
And also make sure the wheel axles are lubricated as these can sometimes not be as free running as one might believe.
And finally check the motion gear. It could be that part of it is out of line and locking when reversing the loco.
I think thats all!
I have operated these motors on a DCC set up in analogue mode and they perform OK normally. Don't know about DCC mode though as never tried it.