iTunes + dns = slow download issues

Welcome Guys.

From few days i have really annoying bug.

My apps are downloading really slowly.

Just check photo: http://puu.sh/jRK7H/ebc2a8f407.png

Normally apps are downloading for ~40Mb/s - 5MB/s.

Don`t know why i have that issue.

After changing stock ISP DNS for google dns/open dns/other dns an issue disappear for 5 to 10 download and i have to change dns again.

I`m on

http://puu.sh/jRKdB/ec5c6e50ee.png

on win 8.1 x64 lenovo y570 laptop.

I have updated iTunes to newest version and nothing happens.

Any ideas how to fix this problem permanently?

Regards