Try looking at the LCD screen when you press a button. I've noticed that when you press an LCD button and it miss fires the button press does not highlight the text which it should do to indicate that the button has been pressed. When mine misfires the button acts as if it wasn't pressed at all.

Would you mind experimenting with yours and reporting back on what the results are? I would really like to know if this issue is an isolated case or one that is common on all the 900s.