Have a x58 mobo with i7 920 processor and are having issues with reported memory, it has 12GB installed but sometimes it shows 12 and sometines only 8GB, if I turn it off and reseat the memory it usually shows 12 until I reboot the computer. pc is about 9mth old.
Any suggestions as to the problem?

tried updating the BIOS?

Has this always been an issue since you've owned the PC?

What is the speed of your RAM? there is known issues with RAM thats over 2000Mhz

Yes, latest BIOS is installed, it has been a problem since pc was first built, not sure of RAM speed but I think its only 1066 or 1333Mhz

Is your RAM configuration three sticks, 4 GB each?

If so, then it might save you some troubleshooting time to systematically test each stick.

That way you'll have a more clear idea if the this is a mobo, RAM stick, or software issue.

Memory is 6 x 2GB sticks

Ah, well. It will take a while, but you could ...
1.) Remove all but one RAM stick.
2.) See if your system consistently recognizes it.
3.) Swap RAM stick.
4.) Repeat steps 2 and 3 until each RAM stick has been tested in the same mobo slot.
5.) Test each remaining mobo slot with a RAM stick you know works.

This way, you'll know if it is a bad RAM stick and which one. Or it may turn out to be a bad slot on your mobo. This method is more precise than using a software-based RAM diagnostic tool, but it does take a while to do all of the steps.

Computer ran fine today for over 4 hours, everytime we rebooted it it recognised all of the memory, memory running hot so will try another fan at the front of the case.
Ran some memory diagnostics and all passed OK.
Not sure if relevant but sometimes when booting up the BIOS detects a new cpu and we have to hit F1 or F2, went into BIOS and everything seems to OK.