Mine had same symptons and eventually wound up being the Head Gasket had blown.

When it happened the Oil mixed with water to make that Creamy white emulsion. This crap had filled and blocked loads of the hoses.
The water couldnt flow freely round the system so my heater would suddenly stop blowing hot. Other days it would be fine.
When this happened it took me months to work out what was causing it. I only eventually worked this out when the car started drinking gallons of water.

blocked heater matrix ?

this happened to my mates astra, sum resistor under the bonnet somewhere!

I can get hot air sometimes then other times just cold air!!

happened to me :(
permanent cold air all thru winter :(
got fixed las month
got thermostat changed

but found out my heater hob was broken no matter way i turn it always cold air so guy put it to hot air for me now its only hot air :mad:
somethings broken round bakc

My heater is playin up. Works sometimes then other times no heat from it at all. All other things work such as blowers, rear screen demister etc but just no heat from the heater.

Anyone got any ideas???

Thanks:wave: