There are different solutions to fix flickering, because there are different things that cause it.
It would help if you tell us which version of Premiere you are using, because the controls may be slightly different. But in most versions of the program, if you right-click on the still and choose Field Options, you can choose a couple of different options that may help: Flicker Removal, or Always Deinterlace.
Here’s an article on Adobe’s web site that may help. It’s geared towards an older version of Premiere, but the solutions are still pretty good and the explanation at the bottom tells you why this can happen.
https://www.adobe.com/support/techdocs/315258.html