I run nightly builds in order to get additional metrics recorded at night. So, I have about 50 builds that all queue up at 11pm every night. The issue is that I will have from 1 - 5 jobs fail at night due to an environment variable not being picked up. It'll run fine if I just rerun the job by forcing it, though. Any ideas why Hudson will basically lose the definition of the defined environment variable (defined in node config in Hudson), but only when during this loaded up nightly run?

Here's the output from one of the jobs:
Buildfile: /usr/local/hudson/workspace/scrs-nightly/scripts/build.xml

BUILD FAILED
/usr/local/hudson/workspace/scrs-nightly/scripts/build.xml:27: /usr/local/hudson/workspace/scrs-nightly/scripts/${env.cobertura.home}/lib does not exist.

Total time: 2 seconds

My build script looks like this:
<project name="scrs-dev" default="build">

<property environment="env"/>

<property name="ant.build.javac.target" value="1.5"/>
<property name="ant.build.javac.source" value="1.5"/>

<property file="ant_build.properties"/>
<property name="output.dir" value="${project.dir}/target"/>
<property name="build.dir" value="${project.dir}/build"/>

<path id="cobertura.classpath">
<pathelement path="${cobertura.home}/cobertura.jar" />
<fileset dir="${cobertura.home}/lib">
<include name="*/.jar" />
</fileset>
</path>
<taskdef classpath="${cobertura.home}/cobertura.jar" resource="tasks.properties" classpathref="cobertura.classpath" />
...

The ${cobertura.home} is defined in the property file referenced right after the environment as:

1. The following are added for Hudson Support
   project.dir=${env.WORKSPACE}
   base.dir=${env.WORKSPACE}/source
   cobertura.home=${env.cobertura.home}

The env.cobertura.home is defined in the remote node configuration this job runs on with:
name: cobertura.home
value: /usr/local/Hudson/tools/cobertura-1.9.4.1