Problem with the heater in a Vauxhall Corsa, 1.2L, 98. Warms up but then goes cold with no heat?

It will blow out warm air but then go cold again and stay cold. If I turn it off for a while and then back on it might blow hot air again.
Any ideas what might be wrong cause its coming into the Winter!
Thanks

Answers:
Air lock in the water system. Will need draining and re filling. Also could be a faulty heater radiator but the first one is the cheapest to look at.
Get the radiator valve changed. It's an inexpensive fix and that is usually the problem.
Seems like a bad heater control valve. If it's vacuum operated, check for a bad vacuum hose. Also check the coolant level in your radiator. If it's low the heater will only work intermittently.
Master tech 40 yrs.
i agree with fred bear, look at the radiator as you might be low in water as the car will stop the flow to the matrix if it gets to low
the heater matrix is blocked, its like a little radiator inside the car which the fan blows across to give u warm air, this is blocked and not flowing enough water to keep u warm, take to garage and ask them to flush it thru
sounds like No more than a thermostat my not even have one in fit a new one any way if that don't work the air lock could be a problem air lock is some times caused by a head gasket on the blink simple things first
It seems like you need to back flush your cooling system

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