certainly helps to see it... Why are you using this specific pixel dimension? I thought Instagram was always a vertical format? What happens if you use one of the export presets for youtube without adjusting the pixel dimensions? Just wondering if you see the same problem. Back in the day, I can remember there were certain rules about placement of videos in a macromedia director project that you needed to adhere to in order to get smooth playback. If I remember correctly, the pixel position had to be divisible by 4. Wondering if there's some voodoo with this particular pixel dimension. Were you following the exact same workflow previously when you didn't see the problem with source and sequence pixel dimensions the same as they are now?
WAIT... did you shoot this material on a smart phone? If so, the problem may be that the sources have a variable frame rate which Premiere has issues with. Unfortunately, these problems are intermittent with a wide variety of manifestations and impossible to predict. Here's how to diagnose and solve the problem
use mediainfo to determine whether your source is variable or constant frame rate
https://mediaarea.net/en/MediaInfo/Download
if it's variable use handbrake to convert to constant frame rate, setting the quality slider in the video panel to maximum
https://handbrake.fr
and here's a tutorial on how to use handbrake
https://www.youtube.com/watch?time_continue=34&v=xlvxgVREX-Y
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