Preview Render not working properly

Hi there, 

Since a while back i have issues with preview rendering on my Imac (40GB of ram). 

ATM is have 2 compositions in one After Effects project. In one comp, preview render works just fine. Every things hits the green line and playback is as normal. In the other comp (where 3 comps come together) it just renders 2/3 frames and abruptly stops.  Leaving just a small bit of green behind and some blue on other places. Sometimes i wait a few minutes, then it works again just to stop working 10 seconds later. It's super frustating as i wont get any work done this way. And this is not the first time i ran into this issue with a project. 

Things i already did: 

1: Purge all memory and disk cache. 

2: Restarting the Imac

3: Restarting program (rarely would do the trick).

4: Removing and reinstalling After Effects 

Hopefully somebody can help me.