Try redownloading mod files, if you're playing through steam, make sure both of you login to the correct accounts and are not playing on the same. Turn your anti-virus software off, check your router settings. You might try setting local static IP adress on both of PC's. Search for a router specific guide or use one of global methods (http://goo.gl/8PQOx) If one of you is using WiFi connection to play, try using cable instead. Check if gets any better and let us know if anything changed. Cheers.