I think it's a good idea to monitor the server with monin, I'll give it a try.
Also i did suspect that the I/O operations were the cause for the high load,
the hdd's of the server are configured as Raid5.
Yesterday the server went "Kernel Panic" and the only thing to do was a force reboot, also with dmesg I'm getting a lot of these messages:
pop3: segfault at 0000000000000004 rip 000000000044333a rsp 00007fff7ffe3a10 error 4 (what can they be?)
The htop is already installed