Connectivity Issues in Minecraft - Desyncing Often and Constant Spikes

The "ever since I built my new gaming computer" is cluing me to issues with your wifi adapter. Old adapters don't particularly have (good) drivers that work well in newer versions of Windows.

Depending on the game you play, you could potentially experience the same issue as well. Minecraft is very sensitive to latency. Other games compensate by things like motion prediction and other things. Minecraft is just Java + UDP ...that's it.

A test would be to ping /t minecraft.server.ip and connect to the server and attempt to play. If you see drops/spikes, then it's your adapter not able to keep up. However, it's also possible that pinging won't do much since it's a different protocol.

Wanted to make this a comment... Not sure how. But in terms of trouble shooting, see if you can't connect the laptop to your wi-fi via the old USB mah'jig. Then try the server, now using the verified laptop, but the suspected USB wi-fi...

That would be my next step in terms of trouble shooting. (That or Ethernet cable right into the new pc then retry. You could set up the machine near the router or modem temporarily to try it out at least. It would seem, anyhow.)

Seems like you did all the other groundwork though. Sounds like it is some tiny subtle issue with that card.