Passed memtest, failed hci design

I have a pair of G.Skill Pi Black 2x2gb ddr2 800 in a maximus formula se @ 1.9v, stock settings. When I first got the pair, I had win xp x64, and I ran memtest 86+ and Hci Design Memtest, and I passed both.

I installed Win 7 RC, and got a new processor [e8400]and I had to give a stick to my bro for a while so he can run his computer.

I started seeing some lag in tf2, which was getting quite annoying. Last time I saw a sharp drop in frames, it was my Ram. So I ran HCI Design Memtest on win7 for about 8 hours only to find out it failed at about 4 hours. I ran it again and it failed again.


Now I ran memtest for stick one for 17 hours, and stick 2 for 11 hours, and they both passed with no errors. Should I be worried about my sticks failing hci memtest?

btw my video card is hd 4850, the lag has been there overclocked or not. I have decent ventilation in my pc, this hot&&humid weather doesn't help too much though.