Heat

[skinner]

Hello everybody.
Century 92 V6 engine.
Problem with heating inside cabin.
Heater core warms up when engine operates but cools down as soon as I move the switch from OFF position to Vent or Heater. This happends in seconds. If I keep my hand on heater core I am able to feel how the core is getting cold when the blower motor starts to pass air thru the heater core. Thermostat seems to normaly operate. Both radiator hoses are hot and the dashboard water gauge works fine.
What could be wrong? The Heater core itself? Coolant flow through Heater Core is not enough for this one to be able to exchange heat but not to get cold?

Thank you.