What appears to be happening on the select Dell models is that when the processor warms up slightly, a throttling mechanism kicks in, cutting the CPU performance by 95 percent or more.  Any running applications are thus slowed to a crawl -- the computer acts as if it is frozen.  The CPU slowly kicks back in, but even after regaining its composure slightly, only reaches about 50 percent of the initial clock speed.

Dell's users have been complaining about the problems on the company's support forums.  In what appears to be a desperate attempt to save face, the company has begun banning troublesome complainers and deleting their posts.  One prolific user that has been banned, "Tinkerdude", has actually release an extensive 59-page analysis of the problems dubbed "Performance loss during normal operation in a Dell Latitude E6500 laptop due to processor and bus clock throttling".  That document can be found here (PDF).

The issues affecting the Studio XPS 1645 appear to be similar, but less debilitating.  Its issues seem to center around an inadequate AC adapter, according to a forum thread.  While Dell hasn't respond to the E6400/E6500 problems, it is shipping beefier replacement adapters to XPS owners who call and complain.