Hello Manivelp,
Are you sure that the system memory pressure is the only thing that is building up? It sounds to me like you are running out of graphics resources. This is very common if you have multiple MediaElements each playing HD video. I would recommend that you use
a tool like GPUView to profile your fideo subsystem once the video turns black. I would guess that you are either running out of vidoe memory or pushing the GPU to its limit. GPU View will help you determine what is happening.
Using GPUView
http://msdn.microsoft.com/en-us/library/windows/hardware/ff570133(v=vs.85).aspx
I hope this helps,
James
Windows Media SDK Technologies - Microsoft Developer Services - http://blogs.msdn.com/mediasdkstuff/