Dotted/gaps in preview line

[hepburns]

I've used After Effects for a couple of years and have never had this problem nor can I find a solution for it. When I try to playback my edit which has no effects yet the green line is evenly dotted like in the screenshot below. 'Cache Before Playback' makes no difference and it doesn't matter whether I'm previewing in Quarter or Full, it only plays with about 7fps even though the composition is set to 23.976. 

I've purged all memory & disk cache, emptied disk cache, restarted AE and my laptop several times but still no change.

Here is what it looks like:

I'm using a Dell XPS 15 9570 16GB RAM and have plenty of storage space.

Hopefully someone could help me fix this.

[Towerguy]

Hi, check your preview settings, it looks like you have it set to preview every 2nd frame.

[hepburns]

Thank you, I've fixed it now. Obviously accidentally set it to that and didn't know how to change it. 

[pranal26730842tqi8]

Can you please let me know where this setting is?

[hepburns]

It's in the 'Preview' window and then 'Skip'