Im on premiere pro 2020, Windows 10, and this problem just came up just a couple of days ago. I've been using premiere pro for monthes and this never happened. Im also importing 1920x1080 60fps clips on the timeline
as your project and timeline become more complicated they can put greater demands on your system. 8 gigs of RAM is the bare minimum and may be part of the issue. You haven't told us your source properties, in particular the codec. If your source clips are h264 you might want to transcode to something like prores422 which although they are much bigger files, each frame does not need decoding by the system for playback... You might also explore a proxy workflow. If any of this is not clear, post back. These things can get complicated.