I think I've finally found the solution to my problem of the AppleTV units dropping out of iTunes. It appears to be a matter of finding the most stable channel with the strongest signal and lowest noise ratio on the wireless router. This is hard (a pain) to test with something like my NetGear router due to a lack of a monitor mode, but it's very easy to do with my secondary Airport Express router since Apple's Airport Utility has a channel monitor mode (click manual and then on the word "channel" and a graph and table will appear with updated monitor conditions for the signal strength, noise level and transfer rates for any devices connected to it.
I went through and manually tried ALL available channels (in 5GHz mode it will only do automatic and it was VERY POOR in that mode compared to the 2.4GHz reception on some channels, but your house/location may vary. Clearly, some channels were doing better than others and since I have two AppleTV units in two different areas of the house, it helped to try them all. I found Channel 10 to have the best signal overall and since switching both AppleTV units to use the Airport Express (by itself, it was still unstable and would drop on Automatic) and changing to Channel 10, it's been over a week since either unit has dropped from iTunes. It wasn't generally lasting a day for the closer unit on automatic. Clearly, the signal level plays a HUGE part in whether the units remain connected to iTunes. Maybe even a temporary signal interruption can cause it to drop? In any case, getting the best possible signal to all units seems to be the key. This might mean moving the router (or adjusting antennas if external) or just finding the best possible channel and bandwidth range for your location. It clearly made a huge difference here. It's never been this stable over the past two years for both units (one would usually drop sooner or later, usually sooner and require either a reset of iTunes or a reboot of ATV (or Finder reset on hacked version) to get them to reappear. Now they rarely ever drop to begin with.
I'm not talking about huge signal differences either making a big difference in behavior. It went from -68 to -70dB on average to -64 to -66dB on average with noise down from -94 to -97dB. Transfer rates vary somewhat over time, but I seem to get over 100Mbit per channel on average now whereas it was getting 68-90 before (My NetGear router often gets 160-180Mbit, but it's a lot harder to monitor the signals over time to find its optimum channel (it also sits in a different location in the room); maybe I can find an app to help with that somewhere?) For most uses, it doesn't make much difference, though. I simply switched my iPod Touch over to the NetGear and the AppleTV units to the Airport Express (was the other way around before; I don't want them on the same unit since a G device can slow down N devices).