Thanks for your help. I now have "show heap status" enabled. After enabling "show heap status", I believe I have only seen the warning occur once. The heap is typically around 65M, with a high water mark of ~125M. It's a bit hard to believe that enabling the heap status miraculously cured the problem, but I didn't have the time to attempt to track down whatever change might have fixed things. Anyway, it is working now. If I run into the problem again, I will report back.
thanks,
galen