I have similar issue too, with 388.1 on my AX86U. Wifi remain connected by not internet connection at intermittent intervals. Even Speedtests for wired LAN & Wifi aren't consistent. Downgraded to 386.7_2 and everything back to super smooth.
My AX86U performance become erratic with latest Asus Merlin 388.1 upgrade. WAN speeds irregular, and wifi connections drops intermittently. Downgraded back to 386.7_2 and everything went back smooth steady.