1990 chevy truck overheating
 

I have a 1990 Chevy 1500 series club cab 4wd 350 v8. The issue is that it overheats but only when the heater is being used. Have replaced water pump, radiator, heater core, hoses, thermostat (twice), and been flushed twice. Also had timing and clutch fan checked. Anyone have any ideas what the problem may be?