|Some sites might be down intermittantly so that sometimes they generate
|bounces and other times they don't.  This explanation sounds lame even to
|me, though.

Though dated, the archives contain the following:

http://peach.ease.lsoft.com/scripts/wa.exe?A2=ind9502&L=lstown-l&P=R10068

which indicates to me that your explanation is NOT lame.

As a matter of fact, I see this almost every day on one of the lists that
I operate.

/Pete