Florian Schmidt:
  Yeah, i got a nice and juicy BUG in it (see below). So
 this is what kills rosegarden regularly here when
 run with RTC timing source. 
That'll be the chap.  Mind you, I never saw the RTC-
based timer measure significantly better than the
system timer at 1000Hz.  Although your measurements
may vary, and it seems, probably would.
Chris