Actually -- your kernel is old enough that it's using
LinuxThreads instead of modern NPTL. LinuxThreads had an irreparable design flaw which led to its abandonment and replacement by NPTL. Under some circumstances, control signals between processes overflow and are simply lost, causing hanging/stuck processes strikingly similar to this. I had the same problem, way way back in the day. Ancient history now, unless you're using a 15 year old kernel.
You need to upgrade your kernel.