Just to get you through the deadline, turn off Open GL and multiprocessing if they're on.
And with only 8 Gigs, I'd leave multiprocessing off... Open GL, too.
Thank you for the suggestion Dave. I've tired this just now but unfortunately the issue persists.
While I did have OpenGL disabled in Preferences > Previews its seems I still had Fast Previews set to Adaptive Resolution - OpenGL off. I've switched this to just Off and now seem to be back on track. I believe I didn't have an understanding of what all "turn off OpenGL" truly meant. I'll continue working and report back wether or not I have any more issues related to this.
What would have causes this issue to appear in the first place? I've been working in AE 5.5 for about 2 years now doing a multitude of different projects but this is the first time I've run up against this issue. Is my hardware just simply underpowered?
Based on the information at hand, it could be caused by a variety of things. You might be using a new kind of footage you never used before. You might be using new effects, or a new combination of effects. That seemingly-problematic still may be really huge, and you might have run out of memory with the old settings. Or any of the above in any permutation.