How to troubleshoot a network that is having random issues?

I've been troubleshooting an issue with a client of mine but I can't seem to solve it.

Problem : internet will randomly disconnect.

What i've done so far :

*Found a bad RJ45 which caused disconnects.

*Found a sonos with a bad ethernet port that caused high latency in the network. Moved that one to WIFI, latency issue solved.

*Swapped a bad modem from the ISP. It wouldn't generate the correct speeds the customer was paying for. After swapping it for a new one that issue was solved

*Found an old 10/100 switch. Replaced that one.

What's left?

I'm about to replace a 24 port switch that's next to the Modem. But I'm not confident I'll locate the problem.

Is there any logging software that i can use that might monitor the network so I can find this issue?