[spenner]

Set the number of processors in the PT Playback Engine to half and see if that helps.

Also, have you tried higher buffers? For some USB audio interface setups, Win7 64 bit seems not to allow running the same session on as low of a buffer as XP; Win7 is more robust in that regard. I think the underflow error is related to data not being read as fast as it is being processed between PT and the audio interface. This causes PT (or the audio interface... Not sure which one) to choke because it can't keep up with the rapid buffer refill needs at the current buffer. Or something like that. A higher buffer may help.