The most likely culprit is your defrost heater, which, if the repair man did a "FIX" it could be broken again. The part is easily "fixable" by reconnecting the filament wire that came apart. If you can, I would suggest you replace the heater yourself, so you know it wasn’t a patch job.
The part is located in the freezer behind the panel. You will find the heater at the bottom of the unit held in place (probably) by two philips screws. You can find the part at most local appliance warehouses or I get mine from appliancepartspros.com, it will probably be part number AP2071523, take it with you to the store or compare on the website appliancepartspros.com.
http://www.appliancepartspros.com/partsearch/model.aspx?model_id=210307
http://www.appliancepartspros.com/part_details.aspx?part_id=3687370&pn_=WR51X442
D.Hickman, January 2007