Well, I erased my hard disk so I had the chance to fresh install Windows and Ableton 6. Again, it went perfect the first few sessions, and then it started to stutter and drop while playing the same songs that it was playing well before.
The same behavior: it starts well, disk activity almost null. It keeps increasing until the middle of the song when it becomes unusable. Like "ta ta, ta da da... OMG I forgot to CACHE ALL THAT STUFF!!" The hard disk lits solid orange and each time it reaches a note in an Impulse track, it gets stuck. It only has a Crash (the cymbal!!) note once in a while and the other 4 tracks are all frozen. When you press stop, the whole computer becomes unresponsive and the hard disk reading like crazy for some 20 seconds. CPU usage is barely above 50 %.
The most incredible thing is that, after freezing the last track, it improves a little, but it still gets stuck when playing a note in impulse, even when it's frozen!!! I didn't measure the memory usage but is must be insane because all the system becomes unresponsive.
Back to Live 5 everything is normal. This is obviously a bug.
Using Athlon 64 X2 3800+ (dual core), 1 GB RAM DDR2 800, 120 Gigs SATA, E-MU 0404 USB 2.0 (also happens with the onboard Realtek), Windows XP x64 Edition SP2