Windows Memory Diagnostic Freezing at 21%

I have new Dell Inspirion 7720-6167 Core i7 3630QM, 8192 MB RAM, Windows 8.

* Dell system test (from BIOS, all tests) - passed.

* Dell PC Checker (bundled windows application) - passed.

Windows Memory Diagnostic (aka WMD aka mdsched, recommended by Dell PC Checker):

standard tests - passed

extended test - freezing at 21% (no progress in ~6 hours).

After it i ran memtest86+ 3 times - no problems detected...

I know that "Windows Memory Diagnostic Freezing at 21%" is a popular issue in internet forums... So:

Is it Microsoft bug?

Should I wait WMD longer (one day or week for example ;-)?

Or It is problem with my Dell hardware?

Dell phone support said: we aren't responsible for MS tools, BUT:
- Dell site says "Dell recommend Windows 8"
- and It is Dell PC Checker, who recommend me to run WMD!