Plutonium

When I enter to the multiplayer, it doesn´t mind if it is in custom games or in public servers, my fps are extremely low(8,9) but in zombies the game runs perfectly.
What can I do to fix it?

now I can´t join any party, it says that VPN´s are not allowed.

I can´t play, it always disconnect me from the server and my pc screen gets dark.
Do I must have the base game purchased? I haven´t got it.