Live 8 has been quite buggy. My 2 cents would be to double check that you have the latest available version. Some (mainly Windows) users have had issues linked to the mp3 coding. Using wav files instead of mp3s would cure this.
Couple other things to try:
Quit all the other audio programs such as iTunes or Songbird when using Ableton if you have them running.
Try unplugging each controller you might use. It’s pretty far fetched but maybe they’re sending some sort of corrupted message. Try one at a time and see if problems persist.
I assume you are not using any VSTs and have reported this issue to Ableton support?
I’ve had the same problem a few times as well as a host of other annoying bugs randomly show their ugly head. I wish I could offer something more constructive than that