1997 Other Nissan Models Evaporator clogging up

My evaporator constantly gets filled of crap, (blocks all the cooling vents) every 50,000 km, and needs replacing. I would like to save the $800 that it costs to be replaced and regassed.
The mechanics tell me that their is nothing I can do (and it is a common problem on the Patrols).
Surely I must be able to get some access to it and blow it out on a regular bases? I regularly swamp between recirculate and fresh air but it has not save the unit from blocking up.
Can some one give me the hands up on this?
Regards