maybe screen choice...
I have experienced a similar problem with cakewalk HS 9 while trying to do this; though mine has never stopped, sometimes the playback while record is jerky though...
What corrected my problem was the choice of view i used in cakewalk, track,console,notation, etc. ... apparantly cakewalk will slow down when it has too much to draw on screen during playback ...if you had say 5 - 10 tracks open in notation view while trying to do this...
i record in console view or track view, and have had no problems since.
hope this works for you