No input on the cdu

Hello,

I sometimes happens that during the flight (cruising and descent) the cdu freezes and no longer takes as input any data that I shall insert.
I enter the data on the scratchpad and when I go to put in input (from the button) disappear from the scratchpad data but do not replace the previous ones.
For example it does not allow me to change the runway nor the procedure of arrival (DEP ARR page), I can compile the page "forecast" and will not let me change the speed below FL100 (page DES).
You understand that even if it does sometimes, the problem is very annoying because I can not manage it the descent they approach.

I use 737NGX base package (800/900) under FSX sp2, with Win 7 Ultimate 64.

Thanks!

Michele Ammirata