Does this only happen in arma 3?
Now this may not be the case but one thing may be that its overheats, it downclock to idk 1ghz (idk exactly what it downclocks too), where it then suddenly isnt fast enough to keep up with the game, you get 100% load on all downclocked cores but as they are much slower now, they cant keep the framerate up.

Just some speculation :P

I had a thomson crapbox myself before.
My solution was to call my ISP and ask for them to bridge it so it only works as a modem and then use my own router.
solved all issues. As a modem it actually works rather well.

This worked on TG789nv which was the one i had.