spacebar preview sporadically malfunctions

Often I will go into finder (sometimes directly, other times via a spotlight search) and open a folder, and therein when i select a file and hit the "spacebar", no preview shows up. It seems to happen every so often. Not only this, but also I often find my arrow-keys incapable of navigating through folders, and the folder-level-hierarchy. Again this is sporadic.

Question 1: why does this happen? (And more to the point, why does it happen so randomly?)

Question 2: how do i stop this?