I have an asus p5kc mobo with E6750 and 2 sticks of Corsair XMS2-6400 TWIN2X2048-6400C4 RAM (tot 2000 mb).
The RAM is supposed to run at 4-4-4-12 2,1v which it does nicely on my system but everytime i cut the power source (ie shut down computer and pull out the powercord and plug it back in and restart) the system wont POST and after restarting a second time I get the message "Overclocking failed!" and I'm forced to run the RAM timings in their "defualt" JEDEC setting (5-5-5-18, 1,8v). No matter what I do I cannot re-set the timings back to 4-4-4-12 2,1v without POST failure and then the "Overclocking failed! message (even though nothing is overclocked in my system).
Only after resetting the CMOS using the jumper switch am I able to set the RAM at 4-4-4-12 2,1v again, but only until the next "poweroff" and I will have to reset CMOS again....
 
Is this normal? If no what can be causing this?
 
Note that with poweroff I mean actually cutting the powersupply to the PSU after shutting down comp (pulling cord or using on/off- switch on back of PSU)

Well it's possible the RAM simply isn't running stably even though it's rated at 4-4-4-12. What if you try increasing the voltage on the RAM to 2.2, maybe even 2.3? If that's the case I would attempt to RMA the RAM and see if new sticks fix the issue. Good Luck, and if i think of anything else I'll be sure to post back.

Edit: Welcome to the Forums! =)