It is probably freezing up internally. Most roof units have a freeze (temperature) sensor to detect that and automatically turn the compressor off while maintaining air flow (fan). When the freeze condition clears, it goes back to cooling again. A freeze up can be caused by already chilled air getting sucked back in the intake (a ducting problem), or by low refrigerant in the compressor or other internal problem.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL