Rails Session Problem

Rails 1.2.3
Ruby 1.8.5
Mongrel 1.0.1
Mongrel cluster 1.0.2 (using 2 instances)
Apache 2.2.4
RAILS_ENV=production
session storage = PStore
Solaris 10 (11/06)

Background: I have devloped an automated test system for the system
which
our group supports. It uses a terminal server to connect to the
serial I/O on the
system and uses a LAN connection to mount NFS shares on the target
system,
connect to the databases on the target system, etc. Employed on
Solaris 10 X64,
using Oracle 10g, Ruby, RoR (admin and control), & DRb (for supporting
a publish/subscribe
system used by the test executive, simulator and evaluator daemons).

I have run a sequence of tests using just one browser session.
The sequence of tests alternates between two tests which I will
call test1 and test2. On the 10th execution which should have been
the 5th execution for test2, test1 was executed instead.

Below is a fragment of production.log and a fragment of
test_exec_controller.rb
showing that what should have been put into session[:sys_test_ids].

Why would this work most of the time but fail occasionally?

Could there be a problem at times with flushing the session objects to
disk
s.t. when the next test is run, the previous test's id was still
there?

Any additional troubleshooting suggestions?