I'm having this exact problem as described in these threads:
My connection will drop out completely with the modem still showing as online. I haven't seen the problem resolve itself on it's own yet, when I notice it the only fix is power cycling the modem.
I'm on the 50 mbps plan and have the modem running in bridge mode connected to my Airport Extreme. I've swapped out my Airport Extreme TWICE in the last few months and the problem still persists. I originally thought it had something to do with my MacBook Air at the time (when I bought it back in June/July of last year), and Apple swapped out the wireless card because there was a known flaw in some, but it continues even past that. When it continued after that I swapped it for a brand new Airport Extreme, and when it continued just days after that I swapped out the Airport Extreme again (returning and getting a new unit).
I'm out of ideas, I've been thinking it's something in my configuration for almost a year now (started summer 2013), but recently the problem has gotten a lot worse. Can happen multiple times a week, sometimes even in the same day. I've swapped out and tested all the hardware on my end and everything checks out. I'm not sure if it's the service to my area (which has been pretty rock solid for as long as I can remember) or the Shaw-provided Cisco Modem.
The other threads point to hardware. Really wish I could switch back to my old surfboard (with DOCSIS 3 speeds), that thing was a tank for years and years.
Sidenote: I've also been noticing odd pings in the last year (same timeframe as other issues), where a connection is usually in the 40-50ms range will jump to 500, 600, 700ms+ for a short period, then return back to normal. This isn't bothering me as much as my connection dropping out though - small glitches/delays in voice/video calls is reasonable, connection blackouts are not fun at all.