How much RAM does your super-fast modern desktop have? AND what is the size of your composition?
If you're getting spaces between the green bar, your RAM Preview options may include skip frames.
If you don't have enough RAM to cache a large (or long) video, you can lower the resolution that you're attempting to RAM Preview (in your comp window if you're using Auto, or in the RAM Preview options). Unfortunately, it means that you'll be trading quality of the video playback for duration of frames played.
You can adjust the exposure of the video in the comp or footage windows by using the Exposure Button
Turn up the value to over/under expose the video, but it won't change the RENDER, just the way you look at it in the window.
You can toggle the icon on/off if you have a certain value you like to keep it set to.
Without proper system info nobody can tell you anything. "grey and blue spaces" seems to indicate there is something wrong with your disk cache and possibly other settings...
...and just because you have "a super-fast modern desktop computer" doesn't mean it has enough RAM for efficient AE use. If you have 16 gigs of memory, you're on the low side, but you can still get lots of good work done.
Plus we don't know anything about the duration of the comp. A minute or so? That's possible. An hour? Forget about it.