ASUS P6T Deluxe v2 - still sporadic disappearing of 2GB of RAM

Hello Everyone,
This and similar problems have been disgussed in many forums and there
are so many different hints of solutions.
Just would like to know if anyone came across one that really works.

My setup:

ASUS P6T Deluxe v2 Mainboard
Intel I7_920 Processor @ 2,66 GHz
BIOS ver. 1202 (if i understand correctly, it's the latest version of
22.12.2010)
OS is Windows7 64bit

RAM memory is 3x 2GB DDR-3 sticks seated in DIMM Slots 0, 2, and 4.
The RAM modules are CORSAIR CMX6GX3M3A1600C

In most cases, when powering up the machine, all 3 RAM sticks are
correctly detected, thus yealding to 6GB of total memory, running at
1600 MHz.
Sometimes however, one stick simply "disappears" and even the BIOS
will show only 4 GB of RAM.
In this case, the BIOS and the OS report 4GB of RAM.
The utility "PC Wizard 2012" will tell me that 6GB total memory are
installed, but will show tha first DIMM slot empty.

Powering down, entering BIOS, decreasing the RAM clock speed to 1033
or 800 MHz, changing all sorts of settings, has no effect, several
times when powering up, only 4GB are detected.

Then, out of the blue, when powering up again, 6GB are detected and
everything is back to normal.
When working with the OS and putting the machine into S3 (standby) and
awaking, it will always tell 6GB no matter how often i put the system
into standby.

Certainly i also tried moving the modules around in the slots.

Aside from this effect, the system is totally stable.

As this problem has often been disgussed, i do not assume that the
hardware is faulty.
Has anyone got a good explaination?

greetings from Vienna

Roland Schweiger