I'll answer the more nuts and bolts stuff that might help other people trouble shoot here.
I only use VC so the TI clock is automatically slaved to the DAW. MIDI sync for TI MIDI and Synth both off. As I said this can go nuts sometimes, but the problem never persists. I use clocked aspects a lot so I really could work if this wasn't reliable for me. I don't use any plugin delay compensation. Like I mentioned earlier I switch buffer for the benefit of other gear, but the TI is happy at 128. I really just installed Ableton and it just worked. If there are any other settings I should check let me know.
One thing though....The OS5 driver really improved things a lot for me. In the past when I had issues I found that previously installed versions of the driver would cause strange conflicts....back when the TI used to be fussy about using the same USB port (...why?....). My TI is no longer fussy about USB ports after OS5 which I assumes was a feature of OS5. I it's not that may be significant, but I don't remember it flashing up the warning about USB ports on installation. I did do a fresh operating system install when OS5 came out so cleaning old drivers out might be a good place to look.
I've posted the more general TI and Live discussion here...