do we have a buggy server?

NL wrote: @ThomasTailor93 yes we also talked about this, we are waiting for BYTM (Steff) to hopefully discuss this with our hosting (he is our administrator).
We don't know all the ins and outs of this, I thought and hoped CPU usage is maybe reserved and shielded per VPS but I guess that can depend on configuration and/or contract type (or maybe a technical impossibilty, I don't know).
Thank you for your suggestion.

Yeah, no problem. In general, a VPS is hosted on a Dedicated Root Server with a huge CPU (let's say Intel® Xeon® Processor E5-1650 v3, http://ark.intel.com/de/products/82765/Intel-Xeon-Processor-E5-1650-v3-15M-Cache-3_50-GHz), 128 GB DDR3 (or DDR4) RAM. The hosting company is virtualizing their system with systems like Hyper-V, Promox, Xen, VMware etc.. They can set up the amount of cores (in our case 8 hard, 12 virtual). They guarantee 4 cores, so they set up 4 cores to your server. On the other hand, you can use (technically) ~3,5 GHz to ~3,8 GHz, but most of them guarantee , let's say 1,4 GHz. If you calculate, this is a 12 core machine. You have 10 VPS, one of them is using 12 cores, some 8, some 4, some 2, some 1. Every VPS, according to their usage, is using 30% of CPU. So if you have ~3,8GHz, one VPS is using 1,1GHz. If you have a CPU usage of 90%, you use 3,4GHz. I hope Contabo limited the amount of VPS on one machine, and I hope their machines are much better build then my example. But as you can see, it CAN depend on your VPS. I hope not. Let's wait for Contabo's feedback.

It's definitely better to host both on separate machines.