Aux tracks are a new feature I think implemented this year.  (?)  To use:
 
Solo one track or a group of tracks, and set their output to "new aux track" (its a choice in the drop down menu).  A new track labeled "Aux" gets inserted below the track(s) you are messing with.  Arm the aux track and record all the other tracks onto the aux.  Then archive the track(s) you just solo-ed. 
 
Example:  I had a 3 part harmony each doubled and panned hard LR (6 tracks).  Each track required its own compression, EQ, etc. to make it all gel. When I got it mixed like I wanted, I recorded all 6 tracks to an aux track, and archived all 6 of those tracks.  Now the doubled 3 part harmony is running off ONE stereo track with NO effects.  This freed up cpu, and is a lot quicker/simpler than freezing a bunch of tracks.

This happened to me and I found that the Abbey Road Plates plugin was causing it. Waves corrected the problem with an update to the plugin so check your plugins.

I saw this today on looped play. Seems like the cpu heats up then it starts... I can't hear my fan :( it use to never stop...

This happened to me a few years ago but I don't think I ever found why.

Well, I think maybe I solved it (so far, so good, at least). I was getting all sorts of "ghost in the machine" spikes, and audio dropouts, and the audio engine was stopping. 
 
I increased my record and playback buffers from 1024 to 2048 and things seem to be going okay so far...