the black dots are keyframes with on-stage content. ie, those are frames where your graphic probably changes. the frames without black dots are places where your animation pauses (very briefly between frame 82 and 85, for example, and longer pauses after frames 94 and 162.
if you don't want some (or all) those pauses, remove some (or all) of the non-keyframes.
When you exported from Photoshop did you use LZW compression on the GIF? If so can you try disabling LZW compression and reimporting to see if your issue goes away? What you depict there looks like updated frames only contain the part of the picture that's changing, which is compression. GIF isn't compressed by nature and each frame should be a whole picture, unless compressed. I'm not certain LZW would even perform spatinal compression but it's worth a shot to see if it does.
Aside that have you tried using Import to Library, then dragging the image from the library into the timeline?