My 95 did the same thing. It would work great for an hour then not work for a week, then work great for a couple weeks then not at all for a day or two - you get the idea. CLICK HERE to go to a thread I started when I fixed mine. It may or may not be your problem but is a way to get started.

The heater core always has a little coolant running through it so the air coming through will be warmer than outside air. There is a fix for it using a vacuum operated valve from another truck - if I can run down a link to it I will post.

Dave / Believer45