Fan not working

I have a 2003 Toyota 4 runner. Periodically the fan for both my air conditioner and heat doesn't work. The fan has been replaced and it worked fine for about 2 months. Sometimes it works and sometimes it doesn't. It is very random other that it seems when it is the coldest temp outside that is when the heat doesn't work. It may happen 1 time per week or every two weeks. Sometimes when I leave work for lunch it works sometimes it doesn"t. Please let me know if you have any suggestions.
Thanks,
Linda