I used to have a "once or twice a day things the internet suddenly gets really flaky from certain hosts" problem. It turns out it was my router, somehow being so crap at DHCP lease renewal that it was wedging the hosts connected to it. Since I was already using a bunch of Apple stuff, I put my router/gateway in bridge mode and plopped an AirPort Extreme behind it, and the problems went away.

Though, given the particular devices you list, the problem might actually be support vs. lack thereof for 802.1q (media QoS). Apple hardware supports it at all price levels; every other vendor seems to see it as an enterprise feature (for e.g. VoIP softphone VLANs) and doesn't bother including support for it in their consumer chipsets.