My best guess is that some memory has become unavailable to A5 after finishing and starting another application. Usually when switching between applications it takes a few seconds for the now current app to get all the RAM it needs from Windows (depending on how much RAM you have in your PC). Another possibility could be a bad display driver that does not free all graphics card RAM after A5 exits, thereby reducing the available amount of memory for the next 3d application you start. Not much I can do but recommending to update all your drivers and software.