Good luck getting it fixed my internet decides to get crappy sometimes too.
Go to Pingtest.net to test your ping. The packet loss should always be at 0% or it will lag in games where you need to connect to the server SIF is one of those games. Ping below 100 is good, below 50 is excellent. I don't really know anything about jitter though.
My connection to Riga a close to me city
It would be good to know where is SIF English version server is located so we can get better results. Here is my connection to Chicago.