My mk1 octy that I've just bought sometimes blows cold air out when it's on hot, I've checked the levels, there is water coming into the header tank when the engine is running, it gets up to temp about the same time as my old focus did and sits at a nice even 90, and all pipes and radiator are getting nice and hot. Anyone know what could be causing this? I have searched but not finding anything with the exact same symptoms.

Apart from the obvious one of needing to bleed air out of the heating circuit, it could be a stuck/intermittent servo flapvalve in the Climatic climate control unit. Does your car have normal air con or climate control? If the latter then post back here and I will tell you how to do a self diagnosis on the unit.

 

More likely to be air in the heater circuit.