slic3r keeps shifting layers

hey everybody, this is my first post, and i have a question that i can not figure out myself. i have a prusa mendel I2 and im using pronterface and slic3r with ramps 1.4. i keep getting a random layer shift, its not always at the same layer and it changes from 1mm off to 2cm off! i tried the suggestions on another forum but im not using any power tools while im printing, or having any friction or overheating problems. i tried upgrading from slic3r 0.9.8 to 0.9.9 but im still having the same problem. heres a pic of 4 seperate prints, all having the shifted layer start at a different time each time, the bottom right one i let finnish all the way through.(pic) any help would be greatly aprecited, thanks in advance!

that is normally a skipped step issue , a mechanical issue with your x axis.

ill try tightening the belts again, but i have gt2 belts and belt tensioners on both the x and y axis. thanks for the quick reply

I noticed that my x axis will skip if the drivers are too hot and also if my filament is getting caught on something during the print. check to see how hot the drivers are when running, then if it still happens, watch when it happens and see if the filament is getting caught.

thanks for the replies, it ended up being the filament and the gear attached to my stepper lose, thanks again