First the bad news:
After experimenting more with Roku devices and working patiently with Verizon tech support, the final conclusion for me was that Actiontec (MI424WR Rev F) is going to overflow its NAT table with multiple devices, especially those, such as Roku, that likely connect to multiple streaming servers on the WAN side (which creates several NAT entries). Even one Roku device, under moderate use, is sufficient to overflow the NAT table every few days. Any other type of streaming device that connects to several streaming content providers will cause the same problem, as will heavy web browsing that visits many different websites (one NAT entry per server visited, keeping in mind one website typically will serve content from many servers.) So even if you don't have a Roku, if the total number of devices starts to go above 10 or so, you will overflow this router's unreasonably small NAT table. The solution, of course, is to keep rebooting the router that clears the NAT table.
Possible good news:
Actiontec Rev G and Rev I both have NAT tables whose sizes are approximately 4 to 6 times larger than Rev F. Both have double the RAM of Rev F, and 4 times and 8 times, respectively, flash memory. Both are 802.11N capable, and both are gigabit routers, so your LAN side of the network may benefit as well as your wireless speeds.
I experimented with a Rev G, obtained privately, and the NAT table problem disappeared. However, this particular router did not have Verizon firmware so you'll lose functionalities such as caller id, remote dvr access, etc.
So...the best solution is to get a Rev G and Rev I from Verizon with their firmware, which will also assure you technical support, etc. Up until now, Verizon will not provide these to plans below 150/35.
However, they are now selling Rev I to those who wish to pay (approx. $115) via:
http://www.verizon.com/fiosaccessories
Verizon tech support recommends those who have problems with their provided router buy this supported version.
I will receive mine tomorrow and will post results after a few weeks of usage.
(I have intentionally not addressed the issue of having to pay for this; I'd rather get it free too. I felt that if this works out, the expense is worth avoiding the hassle of frequent reboots and random interruptions in connectivity, plus I get a gigabit router and wireless-N.)