After reproducing the hangs with a smallish C program, I have finally found the root cause of the hangs. I updated my D865GBF Intel hyper-threaded P4 motherboard BIOS from version BF86510A.86A.0046.P10 to the latest version BF86510A.86A.0061.P17 and the hangs disappeared. This is the only time I can remember that a bug in my program turned out to be a firmware bug! :-)