Finder error after taking screen shot, apple button not working either
OK, lets do this. 

In Terminal run launchctl list to see all programs running. 

Then anything that does not start with com.apple.... 

Quit or Delete and make sure it is not in you Login items (automatically starts when you log in)

Check your CPU load and Memory load in Activity monitor.

Gaming sites like Steam might run limits on your GPU.

Lets close and quit all 3D party apps, like coconutBattery and others.

The goal here is obvious. Free you OS X and see how it runs then.

Since your Finder just crashed, Open the Console and select Errors and Faults, then take screen shot and publish.