I have the exact same problem on a PC. It seems to be common, I posted on the Ableton site and others have confirmed it. It does seem to be intermittent and now I am even having problems rewiring Reason into Cubase SX2 with dropouts. Apparently SX2 should be installed first then Reason before the other programs so try reinstalling Live after SX and Reason if you have it. will be an awesome combo when it works!