Ok, so it's been a week and I'm still on DLM-2 (also tried a manual resync). Time to take action!
I've increased my SNRM target by 1db via the Fritz 'Line Settings' tab:
My stats now look like this:
So I've dropped ~1mbit from my receive line speed, but my errors seem to have dropped quite a bit (FEC is about half of what it was before and CRC errors seem to be less although I've only had it running in this config for a couple of hours so it's early days yet).
I also had an idea last night which I think may help us to work out what Mean Time Between Errors (MTBE) rate is required to transition to a better DLM profile. If we record what our CRC per minute (CRCPM) a day or two during a particular sync period and then make a note of what DLM profile we get changed to (or left on) and we can get enough people involved, we may be able to spot a trend.
What do you guys think about that idea?