Unusual RAM trouble

Hi,
I'm running Windows 7, 64Bit, originally came with 4GB ram, so i did an upgrade and bought 2 Vengeance Crossair 8GB chips = 12GB of ram, after the installation i could see both in bios and system that i have 12GB RAM and been working fine that way for weeks until today after i reboot the computer it started taking up to 40 minutes to load around 20 minutes alone on the Windows welcome Screen.
The Bios check shows 12GB of ram and when it finally did boot into system i could see 12GB as well but it was still taking forever to boot, so i removed the original RAM chips and kept the Crossair RAMS connected (8GB) and the computer booted just fine it was fast and everything, then tried to remove the Crossair RAMS and boot with the 2 original RAMS (4GB) and it booted fine too.. when i put them both together the system goes back to being unbearably laggy

What's going on here? Faulty Rams? the PC was untouched before the reboot nothing was intalled, no upgrades whatsoever, If they can't get along how come they did after they were first installed?

Motherboard:Intel Corporation DH55HC (XU1)
Specification:Intel(R) Core(TM) i5 CPU 661 @ 3.33GHz Clarkdale 32nm Technology.
The additional RAM that i bought:Corsair Vengeance 8 GB ( 2 x 4 GB ) DDR3 1600 MHz (PC3 12800) 240-Pin DDR3
Latest BIOS, All windows updates in place.

 

It does seem odd they played well together for weeks, and now don't. At this point we don't know if it is a RAM module, the memory controller, or a slot. Or maybe a power problem. Is your PSU adequate for the extra load? Is the system free of heat trapping dust?

Is the original 4Gb also PC3 12800?

Try just 3 sticks. That channel won't run in Dual Channel mode but that's no big deal for testing.

You could test with MemTest86+ but I suspect it will pass.

I suspect the same end results as here: http://forums.cnet.com/7723-7591_102-519905.html?messageId=5096819