This sounds alarmingly similar to the problems I've been having for a while. 90006 error, completely random disconnects - sometimes every few minutes, sometimes every hour, or every few hours. Some days will be almost completely fine, others will be completely unusable, some will be in the middle. It's incredibly frustrating, since yeah, it does make it a gamble trying to do anything in a group, since you can lag out at any moment.

I've tried most of the things you have (also coming from Europe, though mainly to the Chaos datacenter), and... yeah, I really hope there's some solution out there that we're missing.