[john.cord]

In my Setup its 1080i. Its a Panasonic Plasma and a Yamaha AVR.

well, those older "1080i" displays are actually 768p (since there's no such thing as a plasma/lcd display that refreshes in an interlaced manner), and convert 480i/480p/720p/1080i to 768p for display, so 1366x768 is what you want to use in Kodi to avoid having the image rescaled multiple times. If 768p isn't an option, I'd use 720p.

Ok, after tinkering i found the problem. It was a combination of bad HDMI Cable and the new HDMI Splitter. Dont know exactly why but it works now. Very strange.