Moved to the After Effects forum. They will be able to help you here.
The first thing you should do to troubleshoot this problem is turn off Render Multiple frames simultaneously in Preferences. The second thing is to turn off disk cache. That should get your previews running as smooth as your system can handle. Then, if you want, go back into the preferences and turn things on one at a time. Both multi frame rendering and disk cache are very hard to get setup properly because system and software configurations are so different.
I'm experiencing very similar problems. When I hit RAM preview it takes between 7 and 30 seconds to start the render process, which is fast once it starts. For large projects this is maddening. I've tried every setting I can think of it has 0 affect on the issue.
I don't expect to get a real solution as over the years I've just learned to live with AE quirks. I'm sure some will want to blame the system which is a 3770k running at (4.4ghz) 32GB RAM, Asus p8z77a-v deluxe board and an nivida GTX 660 2GB hardware enabled (hacked) video card all running on a Samsung SSD. Others apps are lightning fast and even Premiere performs fantastic, but of course as I live in AE it tortures me to no end now.
But no, you are not alone with this. I also get strange behavior at times where dozens of AE windows flash on the task bar for a few seconds before locking in and rendering whatever it is I'm working with. Current settings are NO disk cache, NO render multiple frames. Projects start off smooth with instant previews but when it gets busy with many different clips things get like I've described above. Should mention I work mostly with Canon XF100 footage for HDV projects maybe the codec has an impact.
Media Cache flaking out, no doubt....
My PC setup is similar to yours, 3930K, 32GB, SSD and a gtx 660
I have also hardware enabled (hacked) my video card.
i keep getting an error when opening after effects
but it shows that the ray-tracing is GPU enabled and the CUDA information. two things i did not have before the hack.
In premiere everiting works great with mercury playback.
I have disabled the card in After effects until i find a solution.
I'm having the same problem with RAM previews not working correctly or at all with multiprocessing enabled. Sometimes it works but then it will start having longer and longer delays before starting to render and then it will just not render at all. Merlin's original post was over six months ago so does anyone know if this has been fixed yet? I just got a new PC at work with dual 6 core xeon CPUs with hyperthreading that read as 24 available processors in AE -- but I can only use one???