AT89C5131, problems with X2 Mode

Hello,

I have a program in C and ASM which works properly on the AT89C5131 from Atmel. But I have a weird problem when I've try to use the X2 Mode without the peripherical (still in X1 Mode): The mode works it's nice and not the program and it is (quite) normal.But then when I want to rebuild the same programe except the lign,

MOV CKCON0,#OxFF;

the program doesn't work the same and bugs when specials functionss are called (just for information the function use the timer 0 as extern interrupt). I've tried lots of things without results, and the worst thing is that this problem is repeted with each microcontrollers....All the at89c5131 I had are now useless and I still don't know where does my problem come from!!! It must be from the program but I am not enought proficient in it so....Please a reply!!! Thank you