threadDelay not ending

I've got some unit test code that forks off test processes using the 'system' function and then delays using 'threadDelay' to synchronize with the test process. This has worked fine until I upgraded to 6.4.2, now some of the 'threadDelay' calls never return - it's like they are stuck in limbo. Any ideas? What would have changed between 6.4 and 6.4.2 that would cause this behavior? Thanks, Rich