I'm having similar issues with my Sapphire 7950 OC card after a restart. Only difference is I can overclock but I have lost the ability to use MSI AFterburner to underclock the memory more than 150 mhz less than the core clock. I've tried all kinds of shit and it's bout pissed me off to the max. Pretty much gave up. Installing the newest drivers as we speak, but I doulbt anything changes...at least for the better.
Did you put that special ATI dll file inside the Afterburner folder? For me, I have to restart Afterburner several times to underclock the memory that far on my Sapphire 7950 OC.
Let's say I reboot:
Stock memory of 1250, can move it down to 600
close and restart afterburner
can set it to 300 something
close and restart afterburner
can set it to 155. also shows up in trixx now.