What matters is your complete chain of connections to the server, regardless of what your ISP says you have. Much of those figures are sales/marketing anyway.

Try doing a continous ping to the datacenter (don't do it forever, just while you're troubleshooting)

NA Data Center 184.107.107.176
JP Data Center 202.67.53.202
EU Data Center 209.130.141.243 (or use the NA one, unless you're far east of Montreal)

When you get 'problems' ingame, it'll show up as a longer ping times (say from 50ms going up to 1000ms range) and packet loss. Long ping times means you're slow in dodging the darn plumes while packet loss means you keep losing connection temporarily at the worst possible time. And those are just the numbers to reach the data center, there will be a slight additional delay to reach the actual servers.

Your PC is fine, FFXIV servers are fine and even the ISP connections serving both are fine... but the connections between those ISPs... that's the problem here and those companies selling 'fast connections' are basically buying better connections which you have to pay for, naturally. Perhaps you can complain to your own ISP (and SE too) so they will improve the connection/peering. This certainly affects our gameplay and their business.

I'm playing via some proxies/VPNs...

Osaka - Tokyo: 75-100ms and usually less.
Singapore - Tokyo: 200-300ms
San Diego - Tokyo: 150-250ms

... then I go to Wisconsin (i.e. Cogent) and it's around 500-2000ms or worse. Note, that if I'm using the Sargatanas server (NA datacenter), my connection gets worse when I move from San Diego to Wichita. As in, it gets WORSE, as I get physically CLOSER to the datacenter. Makes sense doesn't it.... -.- It gets better when I'm in Detroit though, but that's a sacrifice noone should need to make.

None of this is under SE's control. Still, I suppose they could persuade (give money to) certain ISPs to improve things (*cough* SOE -> Cogent *cough*) if enough of us are annoyed enough to post here...