use Mdegrain2 chained

Hello

I attempt to restore old familly VHS, it's hard ;-).

I have 80% of my tape where
super = Video.Msuper(pel=2,levels=1)
MDegrain2(super,QTGMC_bVec1,QTGMC_fVec1,QTGMC_bVec 2,QTGMC_fVec2)

give good result
on other, i use
super = Video.Msuper(pel=2,levels=1)
MDegrain2(super,QTGMC_bVec1,QTGMC_fVec1,QTGMC_bVec 2,QTGMC_fVec2)
super = Video.Msuper(pel=2,levels=1)
MDegrain2(super,QTGMC_bVec1,QTGMC_fVec1,QTGMC_bVec 2,QTGMC_fVec2)

I use 2 time chained Mdegrain2.

It's work on all tape, except one (X264 crash when encoding), do you know , where it could come from ?

Thanks