Mystery possibly solved

I was wondering why my chain had worn from perfect to knackered between measuring it at 1,200 miles
and replacing at 1,500 miles.

As I was changing the chains I noticed that the return idler bearings were binding causing the chain
to slip on the idler wheel - this was caused by a washer having been left off. Whether this was my
fault or not I don't know. Anyway, with the washer back in place the idler runs very freely and the
chain runs much quieter. So maybe that was
it.

If the new (SRAM PC69) chains don't last, though, I'm still going to 8-speed.