After flashing RMerlin's firmware, did you do a full reset to factory defaults? If not, you may not be properly evaluating the hardware.
Please see the link in my signature below to get your router to a good/known state. You may also want to refer to the Nuclear Reset guide too from that same link below.
For recommendations on what router you need to replace the usually excellent RT-AC3100, you will need to provide more details on not just the size of the home to be covered, but also the expected performance you seek from such an upgrade too.
Before going down that road though, I would flash RMerlin 384.15_0 (or better yet, the just as stable RMerlin 384.16 Beta 1) and do a full reset to factory defaults, including checking the 'Initialize all settings...' box and after the router reboots, use the following URL to flash the same firmware again (the URL bypasses the setup wizard to save you some time) followed by another full reset. At this point, I would additionally 'Format the JFFS partition on next boot' (make sure to hit 'Apply' at the bottom of that page and reboot the router at least twice in the next 15 minutes) and finally, use the WPS reset method to do another type of 'full reset' on the router.
Next, use the suggested guides to make an initial configuration and leave it like that until it has proven stable (or not). Do not use a saved backup config file. Do not blindly put in any previous settings/customizations until the router is proven stable at this stage on its own.
To be sure, the above may be overkill and all steps may not be required. But all the steps above should be done to really know (as much as possible) if the hardware is really failing or you should be looking at something else in your network setup.
Don't be intimidated by the number of steps. In less than 30 minutes to an hour, you should know if a new router is needed or if your current router just needed to be brought back to a good/known state to perform as you expect.
