A pc has so many things you can adjust that it can be daunting.
Did you state in last post that you were using a wifi connection from pc to modem? Can you use a wired connection? If wifi, its possible that there are too many devices on that same wifi channel. ie: smartphones, home wireless phones, neighbors, new neighbors???????? this is something that can be adjusted if made aware.
the game uses UDP packets primarily. They are not monitored, and if lost, they just disappear. Your pc however sends them at a specific rate, all the time. I guess the game has a time out method on each pc incoming packet streams. times out udp, goes tcp, timesout tcp drops connection.
make sure modem is plugged with a quality electrical connection. Use an Ethernet cat 5 wired connection.
Make sure modem connection to service provider is tight, not hot.
Reminds me, your internet provider might have updated the FW in your modem??????? You might want to go into modem options for a look see??????????
Many modems have a DMZ option, you can set up a specific port/machine so that its outside the modems firewall. You loose protection but its a method to see if its the modems security that's at issue.
you said nothing was changed in machine. ie: is it set to auto update? if so its possible a driver got updated, or a security update is not working with something in your pc. If things are left on auto functions, if you you leave machine on 24/7................
its possible that you just need a different dns provider. this is something you can change in pc and/or modem but it does require a scan run by namebench. Quite simple if you care to look into it. Prolly the internet, out of your control but I learned to drive my pc at its optional level, not be driven, highly recommend it.
luck
ps: I'm no tech person either, but its easy at the level you need, read and google a lot.....