Heater works today ? Maybe

Cold weather is coming and my '89 Shadow 2.5 heater only heats when it wants to. Most days it blows cool air but sometimes it blows modestly warm and gradually cools down within a few minutes, like a small amount of heat built up in the core but can't keep it warm with the fan going. The air blend door cable is working and I've flushed the heater core. Also I've manually operated the coolant diverter valve with no change. The small heater hoses going into the heater never really feel hot just extra warm. Could the small lines going under the intake or somewhere else be clogged ?