FRIDGE NOT COLD?

I HAVE A ZANUSSI REFRIGERATOR-FREEZER --ZX99/5W(SI)
THE FREEZER IS WORKING PERFECTLY BUT THE FRIDGE IS NOT STAYING COLD FOR MORE THAN 10 DAYS AT A TIME. WHEN THE FRIDGE IS NOT COLD ENOUGH THE TEMPERATURE ALARM WILL START BEEPING I WILL THEN DEFROST THE UNIT FOR ABOUT 12 HOURS. WHEN I RESTART IT THE FRIDGE WOULD BE OK FOR ABOUT 10 DAYS AND THEN THE PROBLEM STARTS ALL OVER AGAIN.SO I HAVE TO KEEP DEFROSTING IT EVERY 10 DAYS.DURING THOSE 10 DAYS THE FRIDGE IS PERFECTLY COLD BUT AFTER THAT IT STARTS TO BECOME LESS COLDER BY THE DAY. I HAVE A FEELING THAT THE PROBLEM MIGHT BE TO DO WITH A FAN WHICH I USED TO HEAR WORKING IN THE REFRIGERATOR BUT ITS NO LONGER WORKING.COULD THIS BE THE PROBLEM AND IF SO HOW DO I GET ACCESS TO THAT FAN.
I WOULD BE GRATEFUL FOR DETAILED INFORMATION ON HOW TO GET ACCESS TO THE FAN SO I CAN CHECK TO SEE IF THERE IS A PROBLEM WITH IT.I WOULD WELCOME ANY ADVICE OR SUGGESTION AS TO WHAT MIGHT BE THE CAUSE OF THE
PROBLEM IF IT IS NOT TO DO WITH THE FAN