Please see this toppic: http://www.experts-exchange.com/Software/MultiMedia_Applications/Q_28150555.html
My problem reappeared after an update of raspbmc. I tried to install openelec and used the same deafult settings which always worked (lately I was using raspbmc, before openelec, I'd prefer raspbmc):
... but they don't work anymore.
Result: "video mode not supported" on my tv screen. I have no possibility to attach any other screen in order to see what's on the screen.
This is my config: https://www.dropbox.com/s/2ay8vgnlef9zdow/Overview.pdf
Please advise howto configure raspbmc or openelec (probably a setting in config.txt but don't find which one).