The viscous fan drive has been replaced three times in the last two years (cheap crappy clutches) so there is no way that is part of the problem,

Sounds like you need to find somebody that can diagnose instead of replacing parts, those clutches are almost $1000 a crack and you have replaced 3 in 2 yr's .

I would hook up a set of guages and run the tractor in the field and see if your compressor stays running, I am betting that it quits. Also I would install a thermostat kit in the evaporator core if it doesn't already have one which senses core temp instead of refrigerant psi to make the compressor stay running longer. These tractors were Deeres first 4 wheeler with R-134. You did say you had good air flow, so you must know about that little foam filter at the right lower corner of dash.