XFX 7870 memory issue?

Hi,
i have tried many way to overclock my XFX 7870 video card. I'm able to raise my core clock by at least 100mhz , and my result seems to improve well. But as soon as I try to raise my memory clock to 1225 mhz so 25mhz my result in 3d mark 11 for example drop to 2400pts.... My normal result in 3d mark with no overclock are around 5400 and this seems to be in the lower third to...
A funny fact :
If I down clock my core and memory clock by 25mhz , my result in 3d mark raise up by 100 pts...
See my result....
overclock by 25 mhz core and memory : I have set power limit to +20%
http://www.3dmark.com/3dm11/5444963
downclock by 25mhz core and memory:
http://www.3dmark.com/3dm11/5444803
Stock :
http://www.3dmark.com/3dm11/5191360

I submit a support ticket to XFX, and their only answer was that my number are rock solid, you have a good product and you need veteran skill to overclock a video card... What a nice answer.

I use afterburner 3.2 and I have lastest catalyst driver instal. I also format my win7 twice and perform cleaner sweep before installing catalyst....

Heres my spec :

MSI Z77A-GD65
i5-3570K
XFX 7870 double fan
2x4 gb gskill sniper
ocz vertex 4 128gb

Do I miss something in overclocking? or I should RMA , because of faulty memory?
thanks