Delays for myrinet on short messages

Hi,

There is a program we have here that performs well on a particular test
case on a number of platforms using MPI, including a beowulf cluster with
fast ethernet and mpich. However, for this test case with myrinet we get
occasional long delays from very slow communications. The messages are all
very short. The MPI calls that might be causing the problem are
MPI_Allreduce, MPI_Bcast and MPI_Gatherv.

Has anyone seen anything like this before, and if so, is there a known
reason for this, and a known fix?

Cheers,

Andrew