Screen flashes when Userform is processing the code

I have one userform that is does multiple excel opening and closing for calculation and other reporting activity.

For one submit button I have 6 excel sheets and 3 excel files are open and updated with the internal calculations in the background and then are closed. But during this process the excel files opening closing are visible to user in the userform background. This application is working fine in the earlier version of excel except for 2013 and higher.

I have used Application.ScreenUpdating = False in all the places where internal calculations are processed.
I have used Application.Visible = False so that only the main user form is visible all the time.

I get many flashes of excel opening and closing. I need to avoid these flashes so that user does not feel any internal process happening.

Can anyone help me?