There's a few things that can cause that. There's a couple of Q articles that reference slow information store performance; take a look at Q183924 and Q216076 and see if either of them help with that.
The server may simply be processing more threads than it can handle, too. I have one server that's doing this on a very consistent basis; it's a pretty beefy box, but it's also serving 2500 mailboxes, most of which are open at the same time. The only thing I've been able to do it about it is to move the most unhappy users to another server. The Q articles I referenced helped a little, but not a lot. Performance Monitor indicates no problems with the server, and CPU and memory usage are well within optimal limits.