DNS nameservers revert to older servers

My system: MacBook Pro (Retina, 15-inch, Mid 2015), MacOS Catalina 10.15.6

Problem:

For the last six to eight months, I begin to notice name resolution seems to slow or fail, so I (usually) check the DNS nameservers from the command line, using Terminal. When the issue is present, I run 'scutil --dns' and this returns:

resolver #1
 search domain[0] : myhost.com
 nameserver[0] : 216.146.35.35
 nameserver[1] : 216.146.36.36
 if_index : 4 (en0)
 flags  : Scoped, Request A records
 reach  : 0x00000002 (Reachable)

These are old nameservers with low reliability. I change the nameservers (using System Preferences -> Network -> WiFi -> DNS or scutil) and I get this:

resolver #1
 search domain[0] : myhost.com
 nameserver[0] : 208.67.222.123
 nameserver[1] : 129.250.35.250
 nameserver[2] : 208.67.220.220
 if_index : 4 (en0)
 flags  : Scoped, Request A records
 reach  : 0x00000002 (Reachable)

However, at some point later (no discernable pattern detected), name resolution will again slow or fail and when I check the nameservers, we're suddenly back to:

resolver #1
 search domain[0] : myhost.com
 nameserver[0] : 216.146.35.35
 nameserver[1] : 216.146.36.36
   .....

I have tried changing the nameservers as noted above, I have used 'scutil' to execute these commands:

get State:/Network/Global/DNS

d.show

d.add ServerAddresses * 208.67.222.123 129.250.35.250 208.67.220.220

set State:/Network/Global/DNS

d.show

get State:/Network/Service/XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX/DNS

d.show

d.add ServerAddresses * 208.67.222.123 129.250.35.250 208.67.220.220

set State:/Network/Service/XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX/DNS

d.show

And I have used 'networksetup' with these commands:

networksetup -listallnetworkservices

networksetup -getdnsservers Wi-Fi

sudo networksetup -setdnsservers Wi-Fi 208.67.222.123 129.250.35.250 208.67.220.220

networksetup -getdnsservers Wi-Fi

Although each set of commands work fine when I execute them, eventually the nameservers will revert to the old 216.146.3x.3x devices, sometimes within a couple of minutes, others within an hour or two.

I have exhausted my knowledge of MacOS and my patience, if someone has any idea what I'm doing wrong or what might resolve this issue, I would greatly appreciate a detailed response!

Thanks in advance!