In reply to a post by DFScale:

Aye. Router is on permanently, so router was anomalous on IPv6, but OK on IPv4.
Client is on and off at will and no pings are shown corresponding to expectation.

One of the things I'm hoping to get implemented soon is traceroutes when this happens from us which should help a bit.
My bet would be your router's filtering ICMP for a period of time, like some kind of DDoS detection
]