iMovie doesn't work (as it did in past), Get unwanted blinking color panels.

My iMac screen showing crazy, often blinking, patches of color on the screen. THIS IS NOT NORMAL. After some tinkering, I have established that this happens only when iMovie is open.

More specifically:

For all my programs, the screen and images work normally when iMovie is closed.

This is also true when iMovie is open and minimized.

BUT when iMovie is open to full size, any attempt to click on the top banner (such menu items like File, Edit, etc on the left or those on the right, like the icons for Time machine, Bluetooth, Wi-Fi, etc.) will start the blinking colors.

If I then open a program like Safari or Word, putting it on top of the open iMovie, any attempt to use that program’s top banner (commands or icons) will start the same blinking colors.

In the current situation, I can not use iMovie!  I did delete and then reinstalled the iMovie program. What else can we do? Help me figure out what is wrong and how to fix it, please. 

Allen K.