My 1996 Corsa keeps overheating on and off??

my corsa keeps overheating.the first time the jug which you put the water and coolant in over boiled and leaked I had a leaking radiator as well which I got replaced and then it was fine for a day or two then it went into the red again and the fan was not kicking in I took it back to the garage they started it up and It didn't over heat and the fan can on. OK its working again great, no the next day I took the car from London to Nottingham just as I was just out side Nottingham the needle went into the red again and it over heated I took it to a garage and they ran some tests checked the head gasket the thermostat and the fan plus they did a water pressure teat they told me that they could not find a fault with the car i told them that's not possible they said maybe you may need a new thermostat but they were not sure. so i picked up the car and drove it to work today a the fan was working in the morning but on the way home it started again going into the red please somebody help me!!!