Suggest you try and compare john9527's and hggomes firmware before you load the old one.
Make sure to flash the 378.55 RMerlin firmware first and then flash one of the others to test them (if you're not on a current version right now).
Then, whichever firmware you test from this point (and even if you want to test the RMerlin firmware properly) you need to:
Unplug all USB devices. Reboot the router.
Reset to factory defaults via the gui. Manually and minimally configure the router to secure it and connect to your ISP. I would also suggest you use new ssid's too.
Do not use a saved backup config file.
You might want to try using john9527's nvram save/restore utility instead (and even if it doesn't work to transfer your old settings to the new router, you should still have a text based list of them that you can use to copy and paste to the new firmware.
With the above done, you are not finished.
You still need to set the control channels for each band to the optimum one. Do not use 'Auto' for the channel to be used. For the 2.4GHz band use channels 1, 6 or 11 only. For the 5GHz band, test each channel individually as they do not overlap at all.
I test this by doing an Ookla speedtest and seeing how consistent and fast the graphs are. There is always one channel that is superior than the others (and usually, by a huge margin). I also test by copying a large (1GB or larger) file from a wired LAN connected computer to the wireless client I am testing with. And I do this for several specific areas (so I can repeat the testing exactly with the other channels) of the total area being covered with the router.
First though, make the router as central (in 3D) to the area you need to cover as possible. Everything else will be much easier to adjust for if this is done first.