Problems with hosting a GMod server (+20 Players) without lags

Hey, on my GMod (DarkRP) server i have a problem with lags. It starts lagging (ping goes up to 200+) if there are more than 20 players. I have already tried set the tickrate down, but that didnt work…
The CPU is only used to 70-80% at 20 players.

Here are my Root Server specs:
OS: Linux
CPU: 8x 2,93 GHz (Intel Xeon X5670 Octacore) I know GMod is only using 1 core.
RAM: 4GB
Hard drive: 120GB HDD

I hope anyone can help me, if you need some more information ask me here…
Sorry for probably bad englisch… :frowning:

Now my server was lagging with 16 Players, an then it crashed with this console error: Segmentation fault

What hoster do you use?

Zap Hosting… why?