not too much time ago I wrote (I wasn't the only one) about the "instability" of the renderer with the message "warning render failed".
Yesterday I switched the cable of the monitor from a videocard to the other (I have two gtx580)...that's all!

since now I made lot of different tests at different resolution and kernel type and all goes ok!.
I wrote this post because in the old one no solution seemd to be given...maybe this could be helpful for someone else

Cheers, Paolo