I'm also getting the same issue.
This used to work under CentOS 5, maybe the issue is with the NFS service.
Since version 0.3.1 is more then 4 years old we have decided to move on to the development version of StressLinux 0.7.190
The new StressLinux works fine under CentOS 6.
I will create an updated StressLinux plugin for ERPXE soon.