Ian,
Systematic fault isolation procedures will find your problem every time,
Intermittent or not.  On the low pressure line, coming forward from the pump
under the center console, the line branches off in 3 directions, left, right
and forward.  Suggest you shut off  the pump in the burping mode, plug one
port at time. If the burping stops when you turn it back on, you are on the
right track.  From there, you need to further fault isolate to the main
cylinders, door cylinders or dump valve, using the same technique.  The
forward line includes the dump valve.  If you have the latest version of the
valve, I doubt that it would be the offender but is possible.
Bob Jude