heater problems 305

Heater problem need help. 305 both heater hoses are on top of motor ,nothing on water pump.Changed tstat,heater core.Have hot hoses after purge hoses.Have heat while idleing, put in gear move 20 feet no heat,purge hoses idle heat. HELP

could have a line closing on you under load

have you checked the blend door actuator?

have you checked the blend door actuator?
couldnt it be the door itselfe? not jut the actuator

door closes,