New (only in 4.0c) re-occuring problem found:
if a track is made up of different audio parts then the transition from each part clicks and gaps as it's played - happens each time, regardless of how many tracks, buses, plugins, etc are used.

a quick fix is to separate the audio and put them on their own track. hope this issue gets resolved quickly...

thanks.
(p4/1500, xp prof, echo layla24/latest drivers)