CC1110-CC1111: cc1110 code memory degradation write protect flash

Part Number:CC1110-CC1111

Hi !

I programmed the cc1110, it works fine for about one week without any problem. Then all of a sudden code memory degraded!!  gone away!! If I reprogram the chip cc1110 then it works fine again.

What shall I do ? shall I use write protect boot block  and block debug commands on the flsah programmer tool? 

i.e. question; How code memory degradation can be prevented ?

Please inform me 

Thanks,

Hasan