Well, 16hours in with using the old firmware and all seems good!
My original sync speed with the ECI modem was 27196 down, 8496 up. The initial connection with the ASUS last night was 29446 down, 8453 up. At 5am this morning it changed to 29476 and 8281.
If I had been using the latest firmware, DLM would have kicked in by now.
I have however noticed that my line attenuation figures are completely wrong - i.e 0.1db down seems a little off!
I've also sent a note to ASUS about using older firmware, so hopefully a fix will come soon.
Thanks Simon for the tip.