Tried Linux and ran Play Music Manager and Steam. Both worked fine.
So it's definitely limited to my PC (on Windows). Also reset router settings and the problem is still there.
I really don't see what could be causing it...
What is your idle network usage like?
Also, are there any other devices on the network that may be using up bandwidth? (Smartphones, tablets, other laptops, neighbours, etc)
The only other idea I have is the DNS used. Try switching to googles DNS instead of your ISP DNS.
Or alternatively, if he already changed the DNS settings to a custom DNS, try setting it to automatic.
There is your problem you said it yourself, sure fire fix is backup needed documents/files and do a fresh install of windows. Could be settings/software/virus causing the issue fresh install would fix it 100% without wasting any more time trying any little thing.
For something like Minecraft, do you still experience the problem when connecting to the IP address of a server? My server's not running right now, but my domain is sicklycraft.com , and the IP that translates to is 98.109.190.190. I imagine you'd have problems connecting to a server under a domain name (like sicklycraft.com), but what happens if you were to try connecting to its IP address?