Please tell us your system specs: OS version, Premiere version, amount of RAM, Hardware specs including graphics card and your source properties and sequence settings. And does your hardware meet minimum specs?
try changing the renderer in project settings: general to software and see if that helps
And you didn't tell us your source properties (where did your sources come from? which camera? what are the pixel dimensions, frame rate and codec? ) and sequence settings (make the timeline active and go to the sequence menu and choose "sequence settings" and tell us pixel dimensions, frame rate and codec.