In most cases, the reason for the flicker
that browser cache is disabled (Tools->Internet Options->Temporary
Internet Files->Settings->Checks for new versions ->every
visit to the page). The browser requests the background image every
time, rather than only once (on page load). The request to the server
and the response take some time, hence the flicker
effect. If you use a skin which does not utilize background images, you will see that this effect disappears.
Check out the following help article for more on this.