Any Solutions for 90k errors?

Some nights I start lagging, it gets worse and worse before I finally get a 90k error (disconnected).

Some nights things are fine and some nights they aren't. It is only with FFXIV that this happens... Are there any solutions for this? Routers settings? ISP related?

Well, 90k is a straight up disconnect, nothing more. So it really could be anything. Some people in EU have noted that their ISPs have been throttling the game at certain times thinking it was something else.