Why do van der waals forces sometimes attract and sometimes repel

so if 2 non polar molecules are attracted, it should be temporary right since it may at certain times also repel due to electron movement around the individual molecules

Van der Waals forces occur very momentarily. They are formed when one side of the molecule is slightly negative and the other side is slightly positive. Since electrons are continually moving, there may be one time where all the electrons are on one side of the molecule. When this happens, the molecule next to it becomes the opposite and the two molecules are pulled together. When the electrons change and shift around, the bond is broken and then the same process is repeated. This is why Van de Waals are seen in gases (notably the noble gases) because the molecules are constantly moving and never have a static bond.

I believe this guy does a fairly good job at explaining this with visuals. My explanation is general and this guy does a marvelous job at clearly explaining it and I believe he goes more in depth.

http://www.khanacademy.org/video/van-der…