Need help maximizing hashrate on MSI 7950 - System hangs with low hashrate

Hello, sorry if this keeps being asked over and over. I recently built a rig (Asrock xtreme4 970, sempron 145, and a single msi hd7950 twin frozr), i've been testing it out and tried to overclock it.

I'm new to this kind of thing. I read plenty of guides and a few forums about it and the best hash rate i could get is near 600, if i don't want it to hang every couple hours.

I made a few tests myself and the best options i could come with are I19, TC 22400 (only tested this one and 24000 as i couldn't find other numbers to test for my video card. The other numbers i had were around 8000 and those would cause lots of HW, despite a high boost on the hashrate), w256, gpu engine 1020 and gpu memclock 1250, for an average hashrate of 605kh. This is still enough to make it hang every now and then.

Isn't this kind of low? I'm not sure if i made anything wrong

I'm using cgminer 3.7.2 on Xubuntu. Thanks for the help