my servis fridge keeps freezing up?

my servis fridge keeps freezing up at the back, it has no ice box just fridge. its on the lowest setting number 1. and the seal around the door is fine i think. the plunger thingy at the back is not blocked and the elements at the back are clean. can i fix it myself could it be the thermostat is this exspensive to repair ?
i wouldn't mind but i took a joint of meat out of the fridge and it dropped on the floor it was frozen solid. we had a veggie dinner that day !!!!. it's ruining all my food, and i can't afford a new fridge. HELP what can i do ?