General sim performance problem

Hi there.Wondering whether anyone could shed some light on a general sim performance problem.I find that once the settings are cranked up a bit and, to a lesser extent, a few sceneries have been added, the graphic and sound quality of the sim has deteriorated. The graphics component is manifest in poorer frame rates (generally down to about nine per second), a slight (half a second) periodic freeze that occurs every seven seconds, an occasional two-second freeze, flashes of large purple/grey sections of ground and water that appear to be delays in the landclass display, and large tiled sections of ocean.The sound problems are slight periodic sound dropouts. These last for about half a second and occur maybe every 10 seconds. I don