I tried to set intensity from 10 to 18, all the same, defined GPU and MEM clock manually, same issue. Trying different combinations for days but again the same.
I have [fglrx] module loaded - fglrx 9.0.2 [Sep 20 2012] with 6 minors AMD Driver and ATI OpenCL Drivers 2.8 and ADL SDK 5.0
It is promising that I'm not alone with this problem, tried to search for it but just can't define correctly the search

In case this can help someone else. I had the Exact same problem this user is experiencing . its the Fglrx drivers!!!
I struggled for a week with this and finally asked for help . The Fglrx drivers we are told to install according to a guide. Linuix nube here, so i followed the guide word for word and never got things going . the person helping me had me remove those drivers (FGLRX) and install 13.XX AMD Drivers and Magically all problems disappeared. I could set clocks and finally mine at full tilt!
Been mining non stop for 3 months 24/7 on that Linux install.
Sorry if off topic for this thread , Love cgminer , and if this could help someone else avoid the headache!