Variance in freezer compartment temperature

i have a 3 year old ge (gss201eta) refrigerator whose freezer temperature varies from 15 degrees F to -9 degrees F. is this typical of this model? several websites specify maintaining a freezer temperature of 5 degrees or less. i have the freezer temp control set at 6. i have recently replaced the defroster heater to the newer 2 wire design, without positive results. one repair person hinted at the possibility of a bad computer board.

any thoughts would be appreciated.