I've noticed something at seems odd about some parts of the gear train. Here is what I expect to see at the interface between gears 5 and 6 (6 = final gear driving the hour hand):
Gear 5 is turning anticlockwise pushing gear 6 clockwise.
But often when I look at this what I see is the following:
The teeth of gear 6 are just resting on those of gear 5 inner, with the wrong faces in contact. I can manually nudge gear 6 anticlockwise so that it is on the correct face of the gear and it will then flop back into this position.
If you look in the background, you can see the same thing is happening between gear 4 and gear 5 outer, and this is likely to be the root cause of what happens between gears 5 and 6. Eventually gear 4 advances enough to engage with the correct side of the teeth on gear 5 outer, and gears 5 and 6 also correct themselves. For a minute or two before it fully engages, you can see gear 5 bounce on each tick.
I am still affected by the stopping problem every few weeks so I'm trying to look for anything that could be anomalous; I've already changed the parts from silk PLA to regular, checked the bearings, polished the arbors and dry-lubricated everything, and increased the weight, and I still get this problem.