Chevrolet Repair: overheating, clutch fan, chevy express

Question
I have a 1999 Chevy Express conv.van. Recently it starts to overheat when I run the A/C ? I replaced the radiator last Oct. new intake manifold,new thermostat,new clutch fan.
Please help!!!!
Thanks ,
Brad

Answer
Everything you have done already I would have suggested. Now we have to go for the outrageous.  Here we go.

Try flushing the motor.  I mean a good job where you hook up a garden hose and the whole deal.  Don't forget to pull the thermostat.  

Could the thermostat be a bad one?  I doubt it.  This would act up more often.  

How about this?  The heater core is plugged.  If this is bad, the cooling system will act decent until a load like A/C.  Once the A/C is on the system needs the heater core for more cooling flow.  If you are plugged here, it would act similar to a plugged radiator.  

Collapsing hose?  Bad bypass hose?