I wanted to overclock my 280x. And even if I up the memory for 1Mhz from 1500-1501 my speeds drop to 300/150Mhz and then the screen starts to flciker and the same is if I try to up the clock speed. I've noticed I can up the speed if I run youtube in the background, I've gotten speed of 1100/1700 but after closing chrome its the same problem.
Thats correct the way it is showing. Your overclock speed is only when needed, like playing a game. When on the desktop the card goes into a kind of power saving mode. Start a 3D game and see if the clocks go upto what you've set in your over clock.
I've found out why I can't overclock. I have a Benq XL2411 monitor. And I have 144hz refresh rate, if I select 120hz I can overclock no problem when I turn back to 144hz the problem is back.
Have a similar issue with my 7950 and multiple monitors, if i OC even 1mhz the idle clocks drop to 300/150 resulting in flickering/unstable desktop, they seem to have completely cocked up GCN1.0 cards in Crimson when using multiple monitors or 144hz monitors.