Probably the one-way bearing in the gearbox slipping. I have used the slap method before to get this problem sorted to get me through a race.
They did well to get the car into the Top 10 and then to finish third against such a tough field. It is nice to see but in reality their where several Top drivers who had real bad qualifying sessions that would have been a lot faster. It shows the potential of the R12 and with more testing and refinement it will be a serious constant contender in the future.
I really like the 2 belt option. A lot lighter and more free rolling but changing that long belt is a real pain in the A**.