Video Render issues ( flickering, losing shadows )

hello fellows , i think we need some help.

we’ve beeen experimenting some issues with video rendering in sequencer.
as you can see in the following video:

some objects flicker (?) in a very nasty way
some objects change their texture/shader on camera
some shadows appear or dissapear on camera.

what could be causing this issues? is it a modeling issue? lightmass? lightmaps? etc…

we are currently running a 64 bit windows 10, i7 5820k @3.3Ghz, 24Gb ddr4 with a 980ti gpu. UE 4.15 but we experience this flickering or losing shadows issues since 4.12.