This is most common with people from regions that have ISPs that dont like each other (IE: certain areas in China and US, certain areas in Brazil and US, etc). If one ISP has a rate limit or outright blocking of traffic for a specific region, you'll get a conflict or serious lag from the same people all the time. The problem is that it could be the backbone provider for the country or region that is causing it. So switching to another ISP isnt going to help. The only way I know of to stop this from happening is to use a VPN that converts the traffic from UDP to TCP and just hope that the rate limit or traffic block is only on the UDP side (probably not likely) or hope that the VPN's country isnt rate limited/blocked (many ISPs rate limit/completely block VPN services anyway and if they are doing this for protocols, you can bet your ass they are doing it for VPN services as well). This is just 1 example of how a conflict can happen, there are other causes as well.
As for the seemingly on/off conflicts with random people, that can probably be explained with how internet traffic works. Internet traffic takes specific routes to get to specific destinations. But sometimes if one route is down/unavailable, the traffic will re-route taking a different route. If that new route is in an area the destination doesnt like.... you get a conflict. This is most common with the Asian continent where some areas are highly islamic and apt to block traffic from regions not all that far away from them that they dont like for one reason or another. These types of shenanigans will play havoc with internet traffic that is being routed or re-routed through those areas. I use to conflict with a chick from CZ many years ago and one day, she was actually able to play with me without causing a problem. It intrigued me so we compared a tracert to one we had done just a few days ago and found that her path to me had changed significantly. But alas it didnt last long, the next day it was back to using the original route
So while it wasnt her location/region specifically that was the problem, it was an area between us that was indeed a problem.
Tell them to move to a new country or region that isnt so paranoid about freedom of speech as their current location is partaking in virtual oppression lol I guess the bottom line is that physically moving is probably the easiest fix. It may be that it's not their country/region, but something in between. And making a physical move could eliminate that problem easier than trying to fix it with a VPN or some other means. Who knows, maybe it's their work/school location thats the problem. College campuses (especially here in the US) are common problem spots for war2 gamers.
