This has happened to me when I copied a LOD1 mesh to use as the shadow mesh without reducing it to a single "Material" before removing the UV Mapping lines and having [ShVertices_Frame0] and [ShFaces] only in the LOD0 shadow mesh section. Try this and see if it cures it.
Just it!!
After reducing "Multi-Material" to "Single Material", flashing stops.
OK.
Next...
On Gun's firing (at first fire), IL-2 1946 freezes and PC display picture continues blinking on-black-on-black.
I think if this is in "Shellstart" Hook's vector or position error.
Now "Shellstart" Hook parameter is appropriately one.
I will research default ship's Shellstart Hook parameter like Tirpitz.
So I'm extracting files from default SFSs onto HDD folder, it needs some time.
If you know other causing of freeze, please tell me.