You cannot downgrade to anything under 380_3000 due to a change in the firmware's file image. Also, any Merlin version under the alpha 380.60 version is no good either. However, you can use the firmware recovery utility to flash back to any version you like including DD-WRT or USBTomato. Those steps are:
- Power down the RT-AC87U.
- While powering up the router press and hold the reset button until the power LED blinks slowly. This means the router is now in recovery mode. When power led starts blinking slowly, release the reset button and proceed.
- Make sure to match the IP and subnet mask of your router. For example, if your router's IP address is 192.168.1.1 then manually adjust your ethernet adapter's (yes you need one) IP to something like 192.168.1.10 and the subnet must match exactly, usually 255.255.255.0. The LAN tab will bring you to your router's LAN IP and subnet mask if you are unsure.
- With the router in recovery mode attach your ethernet cable to any ethernet port other than #1 as that is the RealTEK switch and will not work.
- Now use the firmware recovery utility to flash whatever you want.
- Clear the nvram by powering the router down once flashing is complete, then power it back up while pressing the WPS button until the power LED blinks rapidly.
- Reboot.
- Configure your settings manually or by using the nvram save/restore utility from john9527 which you can get here. You will have to use the nvram utility before starting to save your firmware settings if you wish to use it to restore your settings after downgrading.
Good luck.