Hello Tobias_Claren, using a translator, I understand that you are looking for the native resolution supported for your monitor.
According to the URL Panasonic TC-P50UT50 - SMART VIERA 50 Class UT50 Series Full HD Plasma HDTV (49.9 Diag.), it looks like the monitor supported 1,920 x 1,080 resolution.
I would recommend you to install the latest graphic drivers available for your product. Also, make sure you install the INF of the monitor.
If you continue having the same behavior, would you please send me a DirectX diagnostic report? Please click Start, click Run, type dxdiag in the Open box, and then click OK.