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