Can you provide a bit more details? What does it mean "can't play"? Does it simply not start video playback, freezes at start, freezes in the middle of movie, reboots the device or whatever? Do small videos play better? Do the problems disappear with virtual memory disabled? Is the problem always reproducible/often reproducible/occurs rarely?

Well, using wifi for just anything (not only video streaming) causes troubles on my 770. The device becomes rather unstable, applications crash quite often or even reboot the device. So I can't guarantee anything when streaming video (or even local video playback with wifi connection established). I myself stopped using wifi long ago. But that does not necessarily imply that anybody would encounter the same problems, it can be a defect in my device as well...