It sounds to me like your evaporator core is freezing up, which is something the system compensates for with a drain (so water condensing on the evaporator core can drain). 1st thing I'd check is to see if the drain is clogged.

If it is not clogged, I would check with the vendor and find out how their system prevents core freeze, they may pass coolant continuously through the heater core and have a hot air bleed somewhere that keeps the evaporator from icing up. There might be a fail in there somewhere.

Hope that helps.