How do you even downclock it that much? I have an 6950. The CCC doesn't go that low, and with MSI afterburner, anything lower than about 700 reverts to the default 1250...
Well, you can solve this by setting the lowest memory-clock possible (or close) and then restart afterburner, it will then have a new clock-spectrum for you to use. This new one should reach down to 300, if not, repeat.
I'm not sure if you have to patch the afterburner settings file for this to work, there is a thread about modifying the settings file somewhere on the forum but I can't find it since the search-engine seems broken...
Sometimes my MSI Afterburner glitches and lets me move it to 300MHz =D
Maybe you did what I wrote above without knowing what caused the "glitch" yourself?

The card runs its memory at 300MHz when it is idle as a powersaving feature. You are merely forcing it to run its memory at its idle clocks since it is actually idle.
Oh, the other day I wondered why the core- & memory-clocks fluctuated when idling

Thank you for the knowledge!