Constant lag freezes, DCs and bad customer support.

[worldofneil]

Playing in Zodiark and Asura didn't get me a single freeze.

Your avatar suggests you play on Behemoth normally (server being physically located in Canada), Zodiark is physically located in Germany, Europe. Asura is physically located in Tokyo, Japan. They're all in completely different regions, so you have completely different network routes to get to those servers.

Tried another ISP in my grandma's house. Same thing.

You need to do a traceroute to see HOW you get to the NA servers. Trying with a different ISP doesn't mean you won't necessarily take the same (problematic) route to the NA servers, you'd need to check the route to be sure.

It's definitely their servers.

We'd all be having issues if it was and you said yourself already using WTFast pretty much fixes the problem, so that confirms it's a routing issue (SE servers don't change because you use a different route to get to them).