Re: VDPAU hangs/freezes even with 256.52
It sounds like you may have multiple ways to reproduce your issue?
Our next driver release series after r260 fixed a bug described as "Fixed a bug in VDPAU that could cause a 'display preemption' when toggling MPlayer to full-screen the first time." Does that sound like your issue? Do you use the "-fs" (full-screen) option to MPlayer?
If not, or if you do have multiple issues, then please provide an nvidia-bug-report, and details of the other issues you're having. Thanks.