Also, does anyone know if there's debug setting to run Rift in single-threaded mode? Perhaps it's using multiple threads and there is a race condition somewhere that only occurs on some machines and with the timing difference of IS running.

It could also be a memory leak (GPU or CPU) that causes the machine to run Out Of Memory and crash. Check your CPU memory usage in Task Manager. Not sure how to check GPU memory usage without a GPU tool like PerfHUD or something, and I'm not sure that's system-wide anyway...