Graphics Error in Menu Bar on G5 with 10.5.8

Hi Mac friends : On a PPC G5 dual 2 GHz (8 GB RAM, ATI RADEON 9600 Pro Card) following problem : After reinstalling the system 10.5.6 from DVD 10.5.6 2Z691-6327-A 10 times, then executing the Combo Update to 10.5.8 the same error appears : the menus for system preferences, timemachine and Preview open, but I just see a rectangular white area, no graphics. Have found no errors with disk utility, the error comes before and after the combo update, resetting the PRAM did not help, no other problems with the machine. Have removed pref files, checked with preferential treatment - no problems. All necessessary fonts are there. I dont have disk warrior. The install is clean, zero additional programs installed. Does anyone know what else I can do ? If I could access the system prefs from the terminal I could live with it. Thanks a lot !