seems like the Gigabytes and ASUS need less voltage than DFI one. Still havn't seen 4200 ghz with HT and Turbo on with an UT DFI...
With my 3837B395 I'm not able to get 4000 Mhz stable with 1,46V + and low DDR3 speed. System crashes with 191x21 or 200x200 =(
Perhaps i shout get a Gigabyte additionally to check out the differences.