Check your power supply. Mine started to fail by offering lower and lower current until the slingbox fell off my home network. Symptoms as it was dying included poor picture quality with obvious noise on the image, sound lag, the top half of the screen on the botttom and vice versa. Even though the old one still put out 6V when checked with a multimeter, the current was the issue. A new power supply from Farnell solved everything.
Hope this helps.