Have you added setx GPU_MAX_ALLOC_PERCENT 100? That would be your first step, you can either do so in a separate console window, or add it as the first line of your batch file, ie

"set x GPU_MAX_ALLOC_PERCENT 100
cgminer.exe -o soso.com....."

I personally open a command prompt, run the command "set x GPU_MAX_ALLOC_PERCENT 100" then it should say value accepted. Close the command prompt. Open your batch, and try for higher TC values. See what your hash is like with a high TC, -g 1, and higher intensity. According to the list, some of the cards prefer single thread, high TC, high I, and some prefer two threads, lower TC and intensity.

I should add, I do not yet have a 7990 (just ordered one 5 min ago, in the process of searching for info I found your thread, so I want to discuss with you so I have a head start on getting mine configured before it arrives)

I do however have a 7950 which requires similar tweaking so I have some experience with these 79xx.

What is your ASIC Quality in GPU-z?

For now I would say reset back to stock GPU/mem, let's find your best hash rate there, whether it is -g 2 low TC, or -g1 high TC, and then work on your OCs. I have found that works best. Overclocking first can lead you to sometimes have what appears to be stable OC, but crashes or HW errors when you crank the intensity. At stock speed we can get a good read on whether your card is a -g 2 or -g 1