Maybe you got hit by Pludervolt like my garage pc did.
My Garage PC is a modded SFF HP with a i5-2500 and HD-4870, 32" samsung 720p lcdtv, win7 X64. It gets pummeled by the various flaws and extra updates compared with my Llano system.
My i5-2500 now runs at 3.4ghz on all 4 cores giving it a 100mhz overclock on a locked OEM board.
Install Throttle stop. When you get it working like you want. Make a shortcut to the throttle stop program file. Put the shortcut into Startup in your Start\programs menu so it will start with windows. Install CPU-Z and click on the last tab, (About) in that tab click on "Clocks" It will show all the core's clocks. Set Throttlestop to run the cpu at max turboboost for all 4 cores. It will give you a small OC even on a locked oem board.
Plundervolt is the newest one to hit intel cpus.
Plundervolt Vulnerability Yet another security vulnerability has been detailed affecting Intel CPUs by three different European universities. This vulnerability is possible through the operating system’s ability to control voltages and processor frequency and allows the manipulation of data...
techbizweb.com
Yes they can overrun the antivirus and make the os run on the slowest downclocked cores to mess with it.
3.6ghz times 4 is your max speed.
I would like to know what the maximum core speeds are of the i5 6500. I know it runs at 3.6Ghz turbo on 1 core, I just wonder what the maximum speed is of all 4 cores at the same time? Did not find it yet on google... Thanks!
www.techpowerup.com
Also disable power savings for the cpu in windows power management too.
My garage pc has ran nice and smooth since i did this.