That's a lot of CRC errors in such a short time. You might be being effected by the 'spectrum' process on this device, killing that process via telnet is the solution at the moment.
Enable telnet access via the admin settings page, then use something such as PuTTY and connect via telnet to your router - the telnet login will be the same as your web UI login. When you're in then run the following command:
killall -9 spectrum
This will need to be done if you reboot the device, at least until ASUS have fixed the spectrum monitoring process to not cause the stability problems on the modem.
As for your speed, try going to the 'Adminstration > DSL Settings' page, disable 'Dynamic Line Adjustment (ADSL)' and set the 'Stability Adjustment (ADSL)' to '-X dB' (where X is something like minus 3 for example, try reducing it until your actual downstream SNRM is around 6 dB).