Distorted Thumbnails after exporting from Photoshop

Hello,

I have been heavily editing images in photoshop (mainly sharpening them up) and I am having a problem with image thumbnails after exporting . When I click in to the image it looks fine but when they are sitting in the folder the thumbnails are completely distorted ( Pixels are all over the place).

Image Details

- Around 800KB - 1MB in size

- PNGs

- I have a DELL laptop so runs on Windows

* Scary Distorted Thumbnail Image Attatched*

I will also note that when I first export them the thumbnails are fine but maybe an hour later they are distorted.  It doesn't seem to happen with any of my other images so I am wondering if it has something to do with how much editing  has been done to the image or just my laptop/software. (maybe I have to do something in photoshop before exporting E.g Rasterizing the layers).

Please please help if you can 🙂 

Thank you,

Michelle