If you run a pc, especially with a multicore processor, you should always disable core parking, and any other power saving functionality. If you run a tool to detect any deferred procedure calls (Like DPC Latency Checker)you will notice how much difference these things make to the stability of your audio and midi streams. I usually disable all hardware I'm not using while I'm playing music too, like network or wifi cards..
Hope any of this stuff helps, but more than anything I wish ableton would step up and make this a priority over all else. Seriously, without tight, locked sync with external hardware your music will never sound as good as it should. It's critical, and is the difference between what you hear in you favourite records, and the stuff you write yourself that always seems a little clinical, or off in some way you can't put your finger on. Rhythm and groove depend on your instruments being in perfect harmony with each other and if you are unlucky enough to be working with shifty clocks, you may never hear your music sound the same any two times you press play