I have a Whirlpool side by side about 6-7 yrs old and it won’t keep steady temps in freezer or refrigerator.  Repairman put a new thermostat in it and nothing got better. I replaced the “control” board and the defrost cutoff switch and it got slightly better.  Freezer now goes from about -5 to +20 or more and refrigerator goes from about 31 or 32 up to 38-40.  Repairman checked everything else out so I’m thinking it’s a Whirlpool problem-either a poor thermostat or some other poorly made thing in it.  I’m not spending anymore on it and either live with it or replace it——but it won’t be with a Whirlpool or any brand they control.  The reason I found out about the temps is one day things were getting soft in the freezer so I put thermometers in both sides.  I really think it’s been that way since new.