Cable Forum - 125M Any advice on irregular short broadband outages?

I have been experiencing frequent but irregular and short broadband outages for well over six months and need advice

Setup:
- GU16 area code
- Former NTL customer
- Superhub [VMDG480] several years old, running in cable modem mode
- Netgear WNDR3700v3 also several years old, running as primary router and wifi hub
- Netgear F5606 additional four port switch
- 2x Netgear Powerline AV200 linking to
- Netgear WGR614 additional wifi hub
The 8 wired ports connect to:
- 3 Windows 7 PCs
- 2 NAS
- 1 Sonos Bridge
The Wifi supports 2 iphones, 2 ipads, and a Sonos Play 5

Symptoms:
- Periodically when web browsing pages will start to hang while loading and eventually time out. Lots of different sites, not just one. The effect is even more obvious when listening to streaming radio - the music just stops. This happens multiple times a day (so far three times between 8am and 10am) but not in any predictable manner. The service typically comes back within 1-2 minutes
- When it happens, if I log into the main Netgear router, it shows that it's Internet connection is down. If I look at the Superhub lights, I see first flashing blue and solid green, then two green flashing, then the blue goes back to solid and the service returns.
- This all happens on both wired and wifi connections, so I don't think it's a wifi problem.
- I've been experiencing the problem for months, but only today got round to calling Virgin support. They quoted a known problem in my area (F003513157) which was identified a month ago and is scheduled to be fixed in a month's time. Strangely there is a different fault reported on https://my.virginmedia.com/faults/service-status - F0035181802 identified a week ago and due to be fixed in a week. But since my problem has gone on long before either of these faults were identified, and since the service works fine with a good 100Mbits on speedtest.net apart from these outages, I am a bit doubtful that either of these faults is the underlying problem.

Theories:
- It could be a fault with my main Netgear router. But I've got it on the latest firmware, so exploring this involves blowing £100 on a new router
- It could be a fault with the Superhub. But I have no way of getting into it to inspect, because it's in cable modem mode. It's a full half day's work to tear down the setup, diagnose the Superhub in router mode, then re-instate.
- It could be a fault with my Virgin connection. But again, I don't think I can inspect the signal levels because the Superhub is in cable modem mode. I also operate a Tivo and a V+ box, both of which suffer from regular pixellation. I guess I could pull the signal levels from one of those, but I am not sure how applicable those would be to my broadband link.

In the mean-time, I have that nightmare: a frequent but intermittent problem that can't be demonstrated to the support engineers but is causing the broadband to be so unreliable that you can't use it for voice, streaming, or video.

Any suggestions? Thanks in advance.