Refrigerator cools then gets too warm... then cools

I have a Samsung RFG297AARS twincool refrigerator... It has separate evaporator coil on the top fridge portion, freezer is on the bottom. Freezer runs great ... Fridge goes down pretty close to temperature I set it (eg set it at 34, it does 38). It will stay like that for hours, then suddenly the fridge temp drifts up to around 50... Sometime it might go back down on its own, sometimes I intervene to reset so it cools... Took off the fridge panel, no ice on the upper evaporator coils... I assume the upper fan works because it does initially cool into the ‘30s ... Any ideas as to what the problem is?