Internet outages

this is a long shot and a question to se if someone can give me some hints.

i have one customer that do experience outages in there home network several times a day. internet just die for a cupple of seconds.
It is an ISP router, unifi AP, fiber connection, in the network there is SONOS system, chromecast, computers and phones. this is what i have tried so far:
1: ruled out wifi by using the ISP built in wifi, and did also try connect computers via cable.

2: called the ISP and they did several investigations on there end and did not find anything, stable connection to the converter all the time, but to be sure they did change out the router two times even.

3: did put in a “real” firewall (Zyxel Zywall) where i did manually set the DNS because this sounds like DNS right? well, that did work partly, most devices started to work without problems, like TV streaming, but a computer did still experience the outages.
so i did start two pings on that compter, one to and one to the firewall. when internet goes down the ping looses connection to both firewall and internet of cource.
so i do reinstall the drivers for the network cards, no luck there. and i can not se any IP conflicts,

i am starting to go a little bit mad here, could it be two problems? one DNS related and one on the computer that wont work? or am i missing something that should be right in front of my face?

could i logg something somehow to get more data? not that keen on starting a wireshark logg to run for a day and try to find something in that heap of data.

Anny suggestions on what to try or look for much appreciated.

If the computer is dropping local pings locally it could be the switch,cabling, the computer, or the NIC in the computer. Replace each device until the issues gone.

switch and cable are tested, the NIC, if a computer have a regular RJ45 port and wifi can it count as one NIC hardware wise? or is it more often two different? (talking private “gaming” type computer) i have tested both cable and wifi with the same problem.