Heater works good then all at once starts blowing cold air

Might help but probably not. In my considerable experience, when scale and sludge blocks a heater only high pressure flushing has a chance of shifting it and a blocked heater is blocked - not blocked one day and ok the next.

More likely an airlock is preventing circulation, maybe the degassing system is blocked...
Try checking both heater pipes are hot when the heater is blowing cold - if they are the trouble could be with the climate control or a stuck air mixing flap or actuator unit that works sometimes...

I need more info. When you set the vent control to defrost does it direct the air to the windshield. and when you set it to floor does it go there. if yes the control to divert air is working. before replacing the control unit you need to find the lines that feed hot water to the heater and check to see if line coming in to the heater is hot and the line returning from the heater is warm to hot. sometime the heater gets plug. if so just disconnect the lines to the heater and flow water from your garden hose to the return line of the heater for about a minute or so this will flush dirt from the heater. reattach the lines and check for heat. This is a known problem for all cars as they get old.

I'd still check the thermostat. Or basically just replace it. Have you ever drained all the antifreeze and flushed the system? Good time to do both. If the hose's get real hot that doesn't really mean the thermostat open. Wouldn't they get hot even if the thermostat didnt open.? If the thermostat was stuck open you'd would have cold air and low temp gauge. Then maybe hit a pump or something and have the thermostat stick closed and have high temp. Maybe water pump. Check it when you have the belt off. Spin, pull & push the water pump pulley around. I'd replaced my water pump at about 110,000 on a '97 dodge Dakota. My 97 never had a very good heater. Always heated up more after driving. Idle never got it warmed up.

you may have a blocked heater core. you can try flushing cooling system if that doesnt work replace heater core.

Check these three possibilities: One, car has a bad thermostat. Two: Car has a bad heater control valve and three: Car has a vac leak under the dash that controls the heater Flap Valve. If this flapper isn't closed completely, cool outside air mixes with the warm inside (recirculating) air and cause a lukewarm air mixture at the vents.

The heater core is possibly blocked or the temperature adjuster is not working.

First check the controls to be certain that the temperature aduster is working and the flaps/doors that direct the flow of hot air are working.

If they are..........

Check heater hose temp. If one is hot and one is cold, you are not getting flow through the heater core.

Bleed it to remove an air blockage.
If that does not work, flush the system.
If that does not work, replace the heater core.

Karl at topgunwon.com

my 2000 saturn sl blows cold air with the heater on. I changed the thermostat,and checked out my heater core.Still blows cold air.

First you have to identify what causes the problem.
To do this you have to:
1) When engine is cold check the level of coolant. If it is low fill it up.
2) Start engine and check if engine reaches regular temperature. If your engine is not heated enough it means your thermostat does not work properly and should be replaced.
3) If engine temperature is normal, then check the temperature of pipes that go to/from heater core. These pipes are located up the fire wall (easier access is from the passenger side). If pipe/pipes are cold (even engine is worm) it means you heater core is plugged and should be flushed. Keep in mind, flushing the whole cooling system is different from flushing heater core.
4) If pipe/pipes are hot/warm, turn off the fan, switch controller to the hot position, wait for 3-4 minutes and turn on the fan. Note if the cold air blows first or warm air blows first but it cools down in 5-10 seconds. If the air blows cold than it is most probably the problem with blend door or blend door actuator.
5) If air blows warm first and then cools down, then while running engine and remaining the transmission in PARKING position, press on the gas pedal until your engine reaches 3,000 RPM. Check if your heater blows warm/hot air. If it does, then your water pump does not work properly and should be replaced. If the air still cold, I would suggest to flush the heater core.

First you have to identify what causes the problem.
To do this you have to:
1) When engine is cold check the level of coolant. If it is low fill it up.
2) Start engine and check if engine reaches regular temperature. If your engine is not heated enough it means your thermostat does not work properly and should be replaced.
3) If engine temperature is normal, then check the temperature of pipes that go to/from heater core. These pipes are located up the fire wall (easier access is from the passenger side). If pipe/pipes are cold (even engine is warm) it means you heater core is plugged and should be flushed. Keep in mind, flushing the whole cooling system is different from flushing heater core.
4) If pipe/pipes are hot/warm, turn off the fan, switch controller to the hot position, wait for 3-4 minutes and turn on the fan. Note if the cold air blows first or warm air blows first but it cools down in 5-10 seconds. If the air blows cold than it is most probably the problem with blend door or blend door actuator.
5) If air blows warm first and then cools down, then while running engine and remaining the transmission in PARKING position, press on the gas pedal until your engine reaches 3,000 RPM. Check if your heater blows warm/hot air. If it does, then your water pump does not work properly and should be replaced. If the air still cold, I would suggest flushing the heater core.

try to back flush the heater core. and if that dont work check the blend door it see if it is working.