Advice to troubleshoot my network

Hi All:

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Im the only IT person at my work, in charge of the whole IT department. It is a small / medium size network with the following network devices:

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ISP modem

1 NSa 3650 doing firewall and routing

2 Ubiquiti fiber distribution switches

20 Unifi APS

15 Ubiquity 32 port ethernet switches

about 50 hosts

5 Vlans

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Besides that, we have about 20 VMS hosting various services, 2 internal DNS servers and more stuff that I dont believe its relevant to mention now.

Last week, staff started to experience very slow internet transfer rates during browsing (spinning wheel). It seems that the DNS server started failing for some reason, so I went ahead and I changed the DNS preference in the sonicwall and the problem was solved (google dns first, followed by our two internal dns servers). Basically, the Sonicwall stopped syncing the license...what is weird is that it used to work just fine .

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Now, I'm experiencing intermittent drops in my network (lan) (I.e I'm logged into some LAN resource and from one moment to other the connection gets dropped for 5-10 seconds). In order to test I started pinging several devices like the default gateway (sonicwall) and a couple of switches. I found out that every 50 packets I MIGHT get 1 or 2 dropped on the sonicwall, and in some of my tests I got about 10-15 packets dropped in 1 of my distribution switches.

Apparently one of my switches is failing, so my question is how can I find which one? What software / hardware tools do you suggest to approach this issue? Feel free to ask me any questions if you need clarification.

I was thinking in getting a Throwing Star LAN Tap (amazon) + wireshark and do some port mirroring. I have no clue on how to use Wireshark but this will be a good opportunity to learn. Do you think this is a good start on how to troubleshoot this?

I would appreciate any suggestion on how to get this solved. My networking knowledge is alright, Im far from being a pro.

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Thanks!