We've been with Spark for 18 months on a VDSL connection. Starting ~6 months ago the connection began to intermittently drop during the night. We would arrive in the morning to see no DSL sync. A power cycle of the modem would fix the problem. 

It happened a few more times over the course of a few months so Spark replaced the modem (hg659b). The problem persisted but a power cycle usually fixed the problem.

Now over the last 7 days its happened 4 times. A power cycle no longer fixes the problem right away. When booting the modem the WAN/DSL LED will flash and then stop. Then sometime (between 30-90 minutes) it starts working. It does need a restart though to make it work - e.g. it dropped at 9:30pm the other night and stayed down all night. A power cycle in the morning and ~45 minutes later it came back online.

We log a fault each time but when we hear from the tech the connection is working again and they can't find a fault.

They have replaced the modem, remotely tested the lines (our voice services are crystal clear and we've no issues), a tech came to site and found some corrosion and moved to a different pair. The problem is getting worse.

Internally we have a Cat5 tie cable from the demarc to our patch panel. The VDSL is split using a master filter and is terminated on the patch panel and PBX. The modem is bridged to an ASA5505.

What should I be saying to Spark to get this resolved? What's the liklihood the issue is within my building (tie cable, filter, patch panel)? Should I look at replacing these?