Thanks for the tips--I discovered the culprit! I THOUGHT I had disabled everything, but while I had disabled the screensaver at one point, I left some power management stuff on (plus, my fiancée had installed some software that turned the screensaver back on).
Everything's off now (power management, screensaver) and I am running a test game, which has been working fine for the past day or so.
Should be ready to host in a day or so.
