AC/Heat Problems - Blower going, but no air from vents

'98 Grand Prix.

I have an intermittent problem with the heat/ac.

Sometimes while driving long distances, I begin to get very little air coming out of the vents.

The AC/Heat works perfectly most of the time, but it suddenly (usually after 1-2 hours of operation) stops blowing air out of the vents. I can hear the blower going just fine, but only a little comes out of the vents. Changing the direction doesn't help either. It happens for all directions, all volumes (1-5), and all temperatures.

Usually, if I stop the car for 10-15 minutes, it will start working again.

Any suggestions? I can't get a mechanic to diagnose it, since it only happens when I go on extended drives.

The mode valves are vacuum operated, so you most likely have a vacuum leak. The most common place is the hard vacuum line that goes from the engine, around by the battery, and then through the firewall. They get brittle with age and crack, and many times are unknowingly damaged if you've worked under the hood around the battery area.

Thanks for the advice. I'll definately look into that.

How does that explain the intermitten (yet fairly predictible) nature of my problem?

Thanks again!

there is a ball that stores vacuum for use at full throttle, passing etc.... You probably have a small leak, just enough to deplete the vacuum ball of it's stores over time and causing your loss of mode. At high vacuum situations (idle, cruising, etc...) everything acts normal.