Memory drops down to zero causing spikes in method response time

I am experiencing this issue with my application. The behaviour is really weird because and from Kadira platform graphs you can see that first the memeory usage drops to zero, then there is a heavy CPU load and then a really long DB waiting time for ‘login’ method.

Did someone already experienced such behaviour on their app? Or Can you point me to some problem that i might be facing?

Here is the image from Kadira:

Server stats are enough:
4GB of RAM
160 GB SSD

Application log does not indicate application restart.

Thanks for any help.