Do you experience this on several clients ?
Maybe you've created a DNS loop somewhere.
Do you experience "Under-voltage" ?
Does dmesg | grep voltage return anything ?
Is your /etc/resolv.conf pointing at 127.0.0.1 (if yes, change it to a public one, 1.1.1.1 or 8.8.8.8 for the command to initially work)?
Can tail the logs live to see when goes south:
Check logs:
tail -f /var/log/{pihole.log,pihole-FTL.log,messages,syslog}
Check kernel ring buffer:
dmesg -w
Specifically for voltage underun messages:
dmesg | grep -i voltage
Check sufficient disk space:
df -h
Check sufficient ram:
free -h
Check load:
uptime
top
Check correct date/time:
timedatectl
Do you have multiple default routes ?
ip r | grep default
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