Network configuration:
Gigabit FiOS coming into the house via ethernet from the ONT outside to a Quantum Gateway router inside. The router also feeds into the coax network, creating a MoCA network which the TiVo is set up as a client. (Note: This problem also happens with the TiVo set up with ethernet, in which case it was connected directly to the Quantum Gateway. I have no other TiVos in the house (yet) -- the plan is to add Minis via MoCA.
There is a second wifi router (Apple Time Capsule) wired directly into the Quantum Gateway acting as a wifi repeater only, with the Quantum Gateway handling DHCP, NAT, etc. The problem occurs when I'm connected to either wifi network (the dual a/n or b/g network from the Time Capsule sharing a network name or the n/ac network from the Quantum Gateway).
The IGMP Snooping thread didn't seem to apply as I'm able to access the TiVo just fine on any one device at a time.