fans keeps stopping so freezer defrosts?

I have an American style AEG fridge freezer, just over 2 years old - the fan keeps stopping meaning that the freezer defrosts and gradually the fridge warms up. i keep having to throw out all my food. Husband has taken back off and cleaned fan which worked for a while, but problem getting more and more frequent. Am scared to leave it plugged in in case it burns the house down whilst I am at work. Please help with any suggestions/ideas thanks