I've run into this problem before and have basically ignored it until now...my ears are ringing...
I'm running a session with 10+ tracks and a large amount of FX. The CPU load meter only reads around 26% but after for a minute of playing the session 1 or 2 of my tracks will severely jump into the red and distort massively. The distortion, a high pitched ringing, will continue until I disarm one or all of the FX on that track.
Anyone know what is happening? This is killing me!