System (ACPI.sys) is always using about 15-20% of my CPU

I had similar issue with Dell E6540, 4 cores, one core permanently at 100%, process: System, thread: acpi.sys. Thanks to @ThePosey I tried to turn off Device Manager -> Network Adapters -> Intel(R) Ethernet Connection I217-LM. Process usage immediately dropped to 1%. When turned on -> one thread started to work at 100% again. Then I noticed that when I switched Power Options -> Power Plans, CPU usage characteristic changed (one core 100% -> 80%, other core 0% -> 50%, depended on the selected profile). Then I researched the problem further and found out a solution: Network Connections -> Intel(R) Ethernet Connection I217-LM -> right click Properties -> Configure -> Power Management -> section Wake on LAN -> disable Wake on Magic Packet AND Wake on Pattern Match. Process usage immediately dropped to 1%, problem solved.

I fixed it! The problem was the BIOS, I updated it after installing Windows 8 (it was a win8 specific update) but it looks like it contained some bugs. If someone have this problem and needs to revert the update here you can find some bios mods based on the original version but also the stock one: http://forum.notebookreview.com/sony/677079-sony-vaio-s-bios-mod-gt640m-le.html

Also: With the updated bios version I was not able to see my Bluetooth, now it's working. Thanks all for the help given.