It stops moving actual air, or stops blowing the temperature you've selected? If it stops blowing the right temp, start with checking coolant level. If it stops blowing air completely, its probably a blower motor or a mess of other things...
1979 Ford F-250 4x4 351M Standard Cab
2012 Ford F-150 5.0 STX Supercab "Vivian"
1992 Ford F-150 XLT Extended Cab "Ethel"