Heater in 2001 Ford Focus blowing cold air, except when accelerating. Thoughts? Suggestions?

I have a 2001 Ford Focus SE. The heater recently started blowing cold air unless I'm accelerating or driving steadily along at a decent clip (at least 40-45). Anyone have any ideas what could cause this? A fuse? The fan works fine at all settings, the hot air just cuts in and out. Thanks.