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 351M Custom S-Cab Long Bed
2012 Ford F-150 5.0 STX Supercab "Vivian"
1992 Ford F-150 300 XLT Extended Cab "Ethel"