Now I know exactly what happens: if you double the frame to adapt the source to a superior refresh a screen judder appear. May seem like blur but it is more like seeing "double". Found that 30fps console games have this when played at 60hz TV. On MAME the same happens when you double 60Hz games to 120Hz. Frame is doubled (with redraw 1 in cabmame) or waited (which has the same effect as draw it twice) so what I see really is not blur, it is doeble frame effect. In fact nothing can be done except avoid doubling the frame, that is giving perfect 120hz refresh (which involves frame interpolation) or lowring monitor refresh speed to original (60Hz). Maybe strobing the image with a blank frame between may help to avoid double frame artifacts, but will introduce some nasty flicker. Seems it is something not easy to solve.
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