Your highest wait type is CXPACKET, which indicates that you have a lot of (unbalanced) parallelism.

My approach to a situation like this is to set up a trace where I capture Stmt:Completed and a few more events filtered for Duration, for instance > 100 ms. But that is more difficult if the application does not use stored procdures, and even more if they don't parameterise statements. But apart from that, this usually gives me some queries to tune (by rewriting, adding indexes etc.)

And since this problem comes and goes this indicates that there is a certain process that needs fixing.