Lpc2000

I write to the unclear area in the flash, but only try to change 1 to 
0. I try to change only 6 bytes from 512 bytes chunk. 

As I told I change 6 bytes, all rest bytes are FF.

But sometimes another, not first 6 bites, influenced by writing, and 
sometimes even changed, from for example 1B to FF, that will be 
impossible operation.

Any idea?