Softhddevice

I have gone thru settings and haven't figured out how to fix if it can be fixed, on SD channels it occassionally goes from fullscreen to not fullscreen for a few seconds and back to fullscreen, HD channels do not do this only SD channels, not really much of a problem just annoying sometimes. I have settings in DVB at 4:3 letterbox, if I change to pan&scan it isn't fullscreen, center cut out chops off top and bottom of pic a little, 16x9 is about the same as pan&scan. letterbox has perfect pictre but a little glitch of going in and out of fullscreen occassionally?? anyway to fix??.