I'll post more info and some photos when I get chance but for now here's a very brief summary of what's been going on.
So I bought an f5 Sapphire from Cotswold online uk. I returned that because of dropouts in running dynamics data. I've just replaced it with an f5 silver and I'm seeing the same dropouts. I'm using the f5 with an hrm-run, the red one.
I'll post some more thoughts about the differences between the two and some photos of the silver with the leather band when i get chance.
At the moment I'm trying to decide if I can live with these issues or if I'll switch to a 935.