I cleaned the fan, but there was not much dust... of course, most technical specs say humidity should be somewhere between 10% and 90%, so I fear I can't blame the manufacturer. I put some duplo-bricks from my kids underneath now, hope they help to get more air into the system.
Thanks for the further remarks, I installed the vdpau -driver, but can't find any settings for it to enable hardware acceleration. What is the command needed?
vainfo returns this:
libva: VA-API version 0.32.0
Xlib: extension "XFree86-DRI" missing on display ":0.0".
libva: va_getDriverName() returns 0
libva: Trying to open /usr/lib/x86_64-linux-gnu/dri/nvidia_drv_video.so
libva: va_openDriver() returns 0
vainfo: VA-API version: 0.32 (libva 1.0.15)
vainfo: Driver version: Splitted-Desktop Systems VDPAU backend for VA-API - 0.7.3
vainfo: Supported profile and entrypoints
VAProfileMPEG2Simple : VAEntrypointVLD
VAProfileMPEG2Main : VAEntrypointVLD
VAProfileH264Main : VAEntrypointVLD
VAProfileH264High : VAEntrypointVLD
VAProfileVC1Simple : VAEntrypointVLD
VAProfileVC1Main : VAEntrypointVLD
VAProfileVC1Advanced : VAEntrypointVLD