FATAL: Thread create failed: Cannot allocate memory

We had one of our MAC-auth radius server instances hang up with this error at about 0200 this morning. That server receives pretty heavy load, and it's bursty, so we see this a couple of times a day: The maximum number of threads (32) are active, cannot spawn new thread to handle request ...but it does not cause problems. An inability to create a new thread is an entirely different matter though; it implies <max threads are running, the server tried to create a new one, and the OS couldn't allocate a thread. Any ideas how to resolve this? Version is FreeRadius 1.1.6 (only reason we haven't upgraded is change control, it's due shortly)