and that is useful. whether you realize it or not. sometimes network issues are simple, if you see X then Y is the cause and Z is the solution. sometimes there's no silver bullet to a network issue, its multiple indicators that clue you in.
fwiw, I got a bad batch of fs.com 5m DACS recently. every single one of them was a turd, every @#$@##$ one was bad. not saying that fs.com is bad. not saying their products are bad. it happens. Wanna know what I ended up using just to get things to work till I figured it out a permanent solution? Used 3m Cisco DACS from the bay that cost like 5USD each. I buy 'em test 'em throw in a box, pull 'em out when needed. They don't work I cut the end off and bin them. why do I have this box? cause cables don't work sometimes. sometimes they work and then go bad. same thing with spare nics. I get it - hobbyist so no you won't have a box of dacs you can dig through.
use the show inter command from earlier
look at this section in the output:
300 second input rate: 224 bits/sec, 0 packets/sec, 0.00% utilization
300 second output rate: 1976 bits/sec, 2 packets/sec, 0.00% utilization
Especially this part:
7685 packets input, 826324 bytes, 0 no buffer
Received 272 broadcasts, 7404 multicasts, 9 unicasts
33 input errors, 0 CRC, 0 frame, 0 ignored
0 runts, 30 giants
And this part:
502846 packets output, 56757049 bytes, 0 underruns
Transmitted 181923 broadcasts, 319738 multicasts, 1185 unicasts
0 output errors, 0 collisions
you will likely need to generate traffic to/from your target, a continuous ping with a 1400 byte payload would be good. let it run even if it is not always making it. look at the stats while this is happening. Note the numbers before you start and after finish. check the stats while the test is running? do any errors go up?
If you have a return window on your switch closing in - you could return it. get another from a different seller. If you have the same problems, could be a bad switch from the new seller, could be your cable(s), could be the nic.
IMO having a spare DAC is NOT a bad thing, nor is having a spare NIC NOT a bad thing but that's for me. Do what's right for you.