AC-88U Dropouts - better solution

Running BT infinity (business broadband). First month with the AC-88U was fine, over the past few days it's dropping like crazy, 10 mins, a few hours. Stripped it back to basics, swapped the DSL cable and BT swear the line is okay outside. Previously i had a separate modem/router.

So really how do i troubleshoot this?

If i go back to a separate modem/router then what's the current best combination for BT Infinity?

I'd really appreciate advice, knowledge or links to where i can find out...

AC-88U Dropouts - better solution