[f-nsp] Dual management Serverirons flip-flopping

[Chris Good]

We recently acquired some second-user Serveriron 400s (Dual WSM-4, 2*B8GC)
to replace our trusty old Serveriron XLs but since then we've been having
some slightly strange problems.  A couple of times a day the active and 
standby management cards switch over causing everything to hang whilst
it comes back up and the spanning tree reconverges.

On the consoles of the standby module we see the following when it takes over:

> Active( in slot 2) management module's heartbeat stopped
>master arbitrate : primary arbitrator is already there.
>
>BOOT INFO: RESET ACTIVE
>master arbitrate : become primary arbitrator.

On the other module there's nothing on the console until but we do see it getting
kicked into standby:

>SLB-lb03>
>BOOT INFO: RESET STANDBY
>BOOT INFO: Become standby CPU module
>M2 BI Boot Code Version 07.06.02
>Enter 'b' to go to boot monitor ...
>Enter standby loop...


The implication seems to be that the standby module stops hearing the heartbeat from the 
active one so takes over and resets everything.  We're running 9.4.2m which as I understand
it is as recent a firmware as can be run on the WSM4.

Has anyone seem anything like this before or have any suggestions as to where we should
be looking for answers?

Many thanks,

Chris