troubleshooting a build failure under mock

My situation is this: my build system is running Scientific Linux 4.4
(basically RHEL 4) with mock 0.6.12.  It fails to build a particular RPM
when trying to compile a fortran file.  The error issued is 

    mkscrlib.f:381: internal compiler error: Illegal instruction

When I set up mock 0.6.12 on my laptop running FC6 with all updates and
run the same script to build the RPM, it builds just fine.

The target distribution for the mock build is FC6.

SELinux is disabled on the SL 4.4 box.

When I run mock with '--debug', there is a lot of information, but I
have no idea what to look for or how to continue to troubleshoot this.

Can anyone offer any suggestions?

Tony