Ford Escort heater fan works intermittendly

all i can suggest is that you may have a faulty switch but there may be someone else here on the sight that may have come across the problem there selfs

You need to check whether or not there is power reaching the motor when this occurs, if memory serves me correctly you can access the motor from under the bonnet just below the centre of the windscreen. Put a test light across the two terminals to see if the live and earth are present, if they are the motor is faulty, if not the problem lies further back in the system and needs to be traced.