Shutter Effect

Hi guys, just wondering on my sons computer he is running FS2004 and he is experiencing some weird things such as using squawkbox or any other dialog based module, what happens is when say for instance we use squawkbox and the dialog appears in the sim anything under the dialog disappears or the background goes black, if we move it up the screen disappears and when we move it down the screen reappears. My sons computer spec's are 2.6 amd athon, 2 gigs memory ram running a nvidia 128 mb FX 5500 video card and a 350 gig hd. Please see screen shots below, any suggestions are welcomed, thanks for your time.