Bad RAM but no memtest errors?

I will try and keep this simple.
I am using 4x 4gb ballistix sport 1600 ram
I was having lots of memory related problems (freezing, blue screens of death)
I took all the sticks out and used them one at a time (normal usage not memtest)
The last stick i tried gave me bad boots and memory related BSOD (irlq not less, memory management etc.)
The same applies for all other slots on the motherboard, good RAM works in all slots, bad RAM doesnt work in all slots.

Now the thing i dont understand is when i run this bad RAM on memtest 86+ for over 5 hours, doing 7 passes, no errors have come up, so to anyone testing RAM, this stick would seem perfectly fine?
Am i missing somethings?

In Brief;
Can a bad RAM stick still not show errors after 7 passes of memtest?
If yes, is memtest reliable?

Thanks
Shawn

The stick is 4gb :)
and it had 7 passes on it :/ yet still no errors? but when i run the PC off that 1 stick its just error after error. without memtest proof is it still safe to say the RAM is corrupt? or shall i run memtest for even longer?
thanks

Golden said:

MemTest86+ is completely reliable. 8Gb in 5hrs seems too short. Run a minimum of 8 consecutive passes.

Golden said:

Ah OK...a single chip.

You might try running it longer. Can you run without any trouble with another RAM chip in exactly the same motherboard slot?

Yes, as mentioned in my first post the apparently faulty ram stick acts the same way regardless of which mobo slot is used. And the good ram stick, works fine in all mobo slots too. I am just confused on how my pc will not boot properly and give many memory related bsod, yet 7 passed on memtest 86+ will give no errors :/ is there any other tests i can run? Once i see errors, i can happily throw it away :)
Thanks