A/P or Joystick Problems??

Hello everybody,I have a little problem with PMDG 737-700.Randomly, in every part of my flight A/P disegnages automatically and CWS mode (r or P) engages by itself.It happens during climb, takeoff, cruise, etc etc..I know what is CWS mode but I don't know why it engages automatically...I don't touch axes of my joystick, I disabled it also, but there's always the problem...I use Fs2004 with FsMetar to have real weather in my flight...I read old post about this argument but I was not able to find an answer to solve my problem...Another question...sometimes when I'm in cruise with LNAV and VNAV mode armed, VNAV disarmed automatically and I can't arm it again...when i tried to arm it FMC gave me the following message "Invalid CRZ ALT"...I don't know why...Thank you very much for your help...Bye.Marco