Guys does any of you overclock your d9 on rampage formula?

I'm experiencing some real weird behavoiurs and I'm not sure is it ram fault or mobo.

I can get working 575mhz on mem with 1.25v on mch and 2.2v on mem - no problems, 100% stable.

BUT

590mhz on mem with 1.45v on mch and 2.45v on mem gives me plenty of errors in memtest.

My conclusion is that such overvolting doesn't look like it would be memory problem, giving 0.25v more and can't get 15mhz seems weird. What you guys think?