[arm-allstar] WIFI won't stay connected

[Chris Z]

Running a PI 3B+ with a fresh install.
The previous install kept disconnecting from my WIFI so I thought I'd start
over.
Same problem.
Iwconfig reports good signal on the connection, -49 to -60DB typical.
Still every few minutes the system reboots because it lost connection.
If I attach the ethernet cable it stays up.
I considered trying a USB dongle but don't know how to force the on-board
WIFI to turn off and use the dongle instead.
I have one that should work since iwconfig shows a wlan1 if I plug it in.
So, how can I stop the constant reboots?
How can I switch the system to use the dongle?
If all of that finally works is there a good way to make sure the ethernet
port is off?