My expression for it is "incipient Alzheimers" but maybe that's an exaggeration. Sadly, it happens to me much more often than it used to. It's certainly frustrating, and the more you try to remember the word you're groping for, the less successful you are. You have to just give it up and wait. When I do that, the word I want usually comes to mind in an hour or so, but often that's too late.

Have you ever noticed the frustration felt by people who have had a stroke and suffer temporary aphasia? That happened to my grandfather and I remember vividly when he first began to recover speech. After 2 days of being unable to speak, he finally got out, "G-g-g-g-goddammit!"