Just to update on what happened here... Despite a stable connection, Interleaving stayed on my line here for over 10 weeks. I saw a couple of re-syncs during that time but each time it reconnected with roughly the same speed and with interleaving depth the same.

Today finally I saw a re-sync that restored the fastpath connection, so now I'm syncing at 79995 with a max attainable of 84173 and SNR 7dB. So glad that DLM eventually did its job, just frustrating it took 10 weeks after the storm that initially knocked me down!!

Errored Seconds if too high will cause DLM to take stability action, you can have tons of ES without a disconnect. Disconnections are just one metric out of many for measuring stability.

In reply to a post by Chrysalis:

Errored Seconds if too high will cause DLM to take stability action, you can have tons of ES without a disconnect. Disconnections are just one metric out of many for measuring stability.

Right, it was a massive burst of ES that caused the original action by DLM. For the past 10 weeks the DLM has tweaked my connection a couple of times (the disconnects I mentioned) but only today has restored me to Fastpath, despite decent ES stats for those 10 weeks. I'd previously been on Fastpath for a year so seems the DLM intervention was somewhat extreme!!

Btw what is a 'reasonable' ES count for a stable line?

Depends on the DLM profile that the ISP uses.
BT/Plusnet (Speed profile) allow 2880 ES/day, Talktalk (Standard profile) half that at 1440.