MPC8641 based custom board Kernel stuck at 1000Mhz core clock

Hi, I am using MPC8641-HPCN based custom board and able to boot linux at MPX clock 400Mhz and core clock 800mhz. When I am increasing core frequency ie MPX clock at 400Mhz and core at 1Ghz, kernel stuck. Does any body have faced this kind of issue ? Or any idea how to resolve this? Thanks & Regards A$hi$h