No luck with persistence mode, this time just starting a video player froze the system...
Typical "NVRM as fallen of the bus", then:
vlc: segfault at 8 ip 00007fd33cc9a4cb sp 00007fffa2b67b48 error 4 in libQtDBus.so.4.7.4[7fd33cc58000+76000]
NVRM: GPU at 0000:01:00.0 has fallen off the bus.
NVRM: Xid (0000:01:00): 3, C 00000007 SC 00000007 M 00001ffc Data ffffffff
So this isn't the fix for this case. "Nouveau" driver is indeed working with low performances, as is 290.10. I guess I'll have to patch 290.10 for the security hole corrected in 295.40, and hope it keeps working with my Xserver untill I get a new video card or a fixed driver.