Is it possible that during the time you are changing the CS1 and CS2 states that one of the following is true:

• Do you have the E line high while the CS1 and CS2 are changing?


• Do you have both of CS1 & CS2 on to the active level at the same time for short period of time?


• Do you have the RST line at active level
  while CS1 and CS2 are changing?

Any of these conditions could lead to data corruption. Especially the first item.

Good Luck
Mike Karas