Refrigerator side sometimes get up to 65 degrees and at other times it is back down to around 51. The freezer is cold. What should I be checking?

Is the freezer temp good? Is the freezer frozen up? Has it been defrosted and the problem still occurs? Model #? I would start by manually defrosting for a few hours and plug back in see if the fresh food compartment improves. If it does it sounds like you could have a defrost problem,