monitor flickering - macbook pro mid 2015

My mid 2015 macbook pro with retina has random flickering. It started about a few weeks after I bought it. It looks like a thin horizontal white line flashes right in the middle across the screen and sometimes with a slight grey glitch. I noticed it was doing a lot of it when I was plugging in external hard drive and also when I work with Adobe Premiere. I had my computer plugged into a multi socket with a table lamp and hard drives. I decided too try plugging my mac directly into the wall socket by itself and it seemed to fix the issue for some time now. Today I plugged my computer into an extension cord and it started doing it again. Nothing else is plugged into that extension cord. Does anyone know a solution? It doesn't happen too often, but I am afraid it might get worse with time.