New FiOS installs, and any FiOS installation of 100Mbps and above use Ethernet for the WAN. In some cases, 100Mbps can be used with Coax, but Verizon is bumping this plan to 200Mbps or more in some areas and Coax won't do that well.
For dropouts, always test using Ethernet first to make sure the drop-outs are not occurring from there. If Ethernet is fine but Wi-Fi is acting up, start off by disabling the SON feature of your FiOS router. This disables the Access Point Steering and Band Steering features, as not all devices play well with these features. The other element you may want to do is set your router to fixed Wireless channels, in case it is changing channels on you for interference mitigation. Use channels 1, 6, and 11 for 2.4Ghz. Avoid DFS channels for 5Ghz unless you're 100% sure all of your devices support DFS, or you don't mind shoving the non-support devices onto 2.4Ghz Wireless.
One note: If you disable SON, your router will create a separate network for the 5Ghz radio to be used by devices. The name will still be named after your normal network name, but with _5G tacked onto the end. Connect your devices to this name for the best performance.