Premiere keeps freezing?

I've never really had problems with Premiere on my computer before. But now it seems to freeze every time as soon as I click on an imported file. It also often freezes when I hit save and often when I am scrolling through a clip (although I have been able to scroll through one clip, but as soon as I click another it freezes again). 

I have a 2012 iMac, with a 2.7 GHz Quad Core Intel Core i5. 

memory is 8GB 1600 MHz DDR

graphics card nvidia GeForce Gt 640m 512mb

editing black magic braw.

Any ideas?

thanks!