Having a really bad time previewing my composition, as when I just space bar preview the green line stops halfway and stutters badly at this point. The RAM preview button has also disappeared! Driving me insane! Any help/advice would be amazing! I attached a screen grab of the window that sometimes appears.
AE has an all-new preview system in CC 2015 as part of the first steps towards a completely rewritten and modernized internal architecture.
See this page for info: all about previews in After Effects CC 2015 (13.5)
Some of the preview behaviors are going to be changed in the next update to make them a bit better: What’s new and changed in the upcoming update to After Effects CC 2015
That should help.
If you really don't like CC 2015, fair enough; a lot of other people are having problems with it too. As I mentioned, it's got some major changes under the hood and it's not all polished yet. You can still use CC 2014 or CC, or even CS6! How to Find and Install Previous Version of Adobe Apps in CC 2015
If your having playback issues, it might also be that your disk cache is full. Go up to Edit>Purge>All Memory & Disk Cache.