If I were you, I would first diagnose in more detail what "locks up" means.

It could mean ethernet packets are not getting from one computer to another
(pinging each others IP no response).
It may mean DNS stopped working.
It may mean your DSL modem is not working, but otherwise the LAN is working.

This is easy to do and would narrow down the device that is causing problem.

So far I know (SFIK -- hey! A new acronym!) this:
The computers in this office utilize a medical records system whereby one of
the computers hosts a server application (which contains the data base), and
3 other computers run client applications which log into the server app and
access the patient data base.

Several times a day the client computers suddenly can no longer log into the
data base (their first indication that something is wrong) and also cannot
access any web sites.

As yet I have not been there when this happens, so cannot confirm ping
success or failure. They've learned to turn off the switch to the power strip
that supplies the routers and modem, and after a few seconds, all is well
with the world again. It's a bit much to ask them to unplug a particular
router's "brick" from the power strip, plug in again, and troubleshoot from
there. I'll have to spend a day there, sometime soon, to experience it
myself.

The next step, I think, is to ask the power utility (PG&E) if they will lend
a power quality recorder.

Thanks,
Sparky