If this were a common occurrence I would suggest to reboot your Tivo(s) along with your PC and your router.
I would suggest to shutdown each in no particular order, but restart with the router first, then the PC, then finally the Tivo(s).
But, since you say it is intermittent, is your network wireless?
Network interference from a neighbor's wireless network would not be unheard of.
If you use a wireless network, try changing the channel it operates on.