Hi Rob and all,
I had the same problem with my model of Plymouth.
Very frustrating to say the least, I must have stripped down the loco and valve gear/motion 2 or 3 times before someone told me about the idler gear. Once I knew what to look for I could see that it was indeed a split idler gear.
At low speed it wasnt noticeable but as you increased speed the running got worse due to the split in the gear opening up under load.
Others with more knowledge may be able to comment but I understand this was a common problem/qc issue with this particular model.
I also ordered a new one from Peters Spares (just a satisfied customer) and have had no problem since.
Good luck,
Regards Glenn