The image will always disappear when you press ANY buttons on the camera as it thinks you want to go back to shooting...(any buttons OTHER than Play, and the zoom buttons, or the dials on top and back...these are the only ones used in playback mode..afaik)
My bet would be you are pressing a button, whether on accident or whatever, and it is simply switching back to "capture" mode.. 
I could be wrong!
also, if you have the AF lock button switched to the zoom out button(one of your custom functions) you cannot use the zoom out button during playback as it will also turn the camera back to 'capture mode'..
maybe this is what is happening?
Good luck