dh overrun

[utzoo!decvax!harpo!npois!ucbvax!michael]

We have started running with mapped out buffers (512 byte blocks) and
get dh overruns when the systtem is loaded.  We have 5 dh's and
are running mapin at spl5.  We also have several non-humans connected
at 9600 baud through the dh's.
I have inserted code which does two iomoves for each buffer, calling
mapout in between.  Im not sure if this has fixed the prbolem
as it has only run for one day.  I think the real killer is in
exece where 512 (1024) fubyte's are done at spl5 if you have a large
argument list.   I have written code to copy arguments to/from an internal
buffer and then to/from a disk buffer in a large group.  I don't
really want to run it because of the overhead.
Has anyone else had similar problems?