My Maytag MSD2356AE gets warm and does not refrigerate properly?

I puchase my Maytag back around 2001 and recently the dispensor completely stops working and now the temperature inside the frig is warming up and will not cool down. I stop the refrigerator and open up the cover behind the freezer section and the cooling coil is complete iced up behind there. Once I completely defrosted the ice behind there and started the refrigerator again the temperature seem to be back down to proper temperature level but that only works for two weeks or so. Now the temperature is back up and everything is spoiling. I am suspecting it is the self defrosting function that is not funcitoning properly. Can anyone help me on which part should I be ordering and how do I replace them? Thanks in advance. Greatly appreciated with any help at all.