Players freeze every couple of hours/days

We have Raspberry Pi 3b and 3b+ players.  They all work fine initially, but will freeze and quit responding every couple of days, sometimes hours.  We initially had them getting dhcp addresses, but I have now assigned static addresses.  I have seen the post about adding arm_freq=1200 and sdram_freq=450 to the /boot/config.txt file and we did that, but are still getting freezing.

When a pi quits communicating with the pisignage console, I can still ping the pi at it's static IP address.  If I try to ssh into the pi, I get a login and password prompt, but then the ssh console freezes.  The only solution we have found is to unplug and restart it.  Sometimes this works, but sometimes we have to re-image the sd card to get it going.

Since I am still able to ping after it freezes, that is telling me it is not completely locked up, But I cannot log in or issue any commands from the pisignage console.

We have tried different SD cards, different Raspberry Pi computers, and putting them on different VLANs.  It seems to work, but then hours or days later it freezes again.

Does anyone have any other things we could try?

Matt