If it was working before, maybe somehow a macro or a keymove was programmed into that key. Try clearing both out of the video source button.

To clear a keymove:
1. Press and release TV
2. Press and hold SET until the remote blinks twice
3. Type 9 9 4
4. Press "Video Source" TWICE
The remote should blink twice to confirm

To clear a macro:
1. Press and hold SET until the remote blinks twice
2. Type 9 9 5
3. Press and release "Video Source"
4. Press and hold SET until the remote blinks twice

That should restore the original function of that key.