What computer operating system is involved? Have you updated 12 to 12.1 with an opened project's Help Menu/Updates.
What are we looking at here - a video clip (to the right) highlighted - and some stills to the left of the video clip or something else? If so, how many frames of the video are affected - just the ones at the beginning of the clip or all?
Export your content to a file saved to the computer hard drive to determine if the issues that you see in previewing exist in the export.
Depending on the above results.....
Switch to the Expert workspace -
Are you working with the rendered or unrendered Timeline? Please press the Enter key. Do you get a rendering dialog or just playback of the Timeline content. If you get rendering dialog, are the problems gone when you playback the Timeline rendered content in the Edit area monitor?
If the problem is not gone, then do this in the Expert workspace, go to Edit Menu/Preferences/General and remove the check mark next to "Show All do not show again messages". Go back to the Timeline, delete you video (if that is the target of your issue), and then re-import it into the project Video 2/Audio 2.
You should get a Videomerge dialog. First, place a check mark next to "Do not Show Again" option and then click No.
Now determine if your Timeline rendered video still has problems.
Any quesions or need clarification on the above, please ask.