Sounds like your TV is in demo/shop mode. To correct this, you should change the source/input to 'TV', then go to Menu>Settings>Plug&Play and find the selection for store demo/home use. Select 'Home use' and your unit will stop changing setting every 30 minutes.
Hope this helps.
my Samsung LN46A530P1FX2C reverts to 16:9 setting after 30 minutes regardless of which other setting I use.