http://bodyodd.msnbc.msn.com/_news/2011 ... mory-clean
The closest thing to an explanation researchers have for this sex-triggered amnesia is that the problem may not begin in the brain, but in the neck. In a January 2010 study published in the journal Stroke, Ameriso and his colleagues conducted sonograms of the necks of 142 patients who'd experienced transient global amnesia within the last week. They found that 80 percent of the patients had what is called insufficiency of the valves in the jugular vein.
This vein, which runs down the side of the neck, carries spent blood from the brain back to the heart. Valves in the veins prevent blood from flowing backward toward the head, but if the valves don't close sufficiently, blood could seep back upward.
The best guess for what might be happening is that patients unwittingly trigger the transient global amnesia by raising the pressure inside their abdomens. This is called the "Valsalva maneuver," familiar as the "bearing down" people might do when lifting weights, defecating or even having sex. The increased pressure increases the resistance to blood flowing down the jugular veins, and insufficient valves may allow deoxygenated blood to push back up the neck. Oxygen-poor blood then "piles up" in the veins draining the brain, especially in central brain regions that are key to memory formation. The result could be transient amnesia.
EDIT: sorry just saw it mentioned in a few other posts: search.php?keywords=amnesia