folie

It is definitely a CPU bottleneck, you yourself said the CPU usage is constantly at 100% and that's what I would expect with a quad core i5, when the game has a spike in CPU intensive work you have no headroom and your frame rate will sharply drop as a result.

Definitely your CPU, I would also check if you have dual-channel memory or single-channel memory. If that is one of the upgradable Clevo's you could slot in a 6700 and another stick of RAM if you're running single-channel and the performance should be significantly better.