Why is the sky blue? Oh, wait, no. There is an answer for that! Humm. Don't know. You shouldn't think of these messages as errors so much as incredibly annoying warnings that something with an almost zero change of happening could potentially be destroying the integrity of your program (http://www.idlcoyote.com/...ips/underflow.html). Most people just grit their teeth and pretend they never see the messages. After a while, this literally becomes true.