heater sometimes blows hot air,then goes stone cold?

last xmas i had a new head gasket (skimmed) ,water pump,thermo,in my car.it started emptying of water 3 wks later and i took it back and they said it was settling in? all year it has been like this,ive taken it back 3 times and the man is busy.I just had a new radiator in my car and another thermo in it as im at my wits end and dont no what else i can do.if the heater run warm i no im going 2 b okay with my journey,if it runs cold the engine over heat,the fan will b constantly on .and it will lose coolant?the 2 hoses by the radiator dont seem to have a lot of pressure in them? but the one at the back has loads in it? its just something inbetween? its a fiat punto 2001