Bob
To eliminate a fault with the fridge thermostat or thermistor and identify whether or not the cooling unit in your existing fridge is failing
Disconnect the electrical heating element/s from the control board and connect them to an extension cord feeding them 120 volts
Over night the fridge cooling unit will work at full capacity and should cool the freezer to below 0*F and the fridge below 35* F
If not the cooling unit has a problem
Remember if you change out a cooling unit the controls don't come with the new cooling unit so it's a good idea to know if they are working or not before
Replacing the cooling unit.
ARP fridge defend has a very good web site to help you understand how your fridge works and help you diagnose and repair it.
https://www.arprv.com/norcold-troubleshooting-repair.phpThe fridge defend product is designed to shut the fridge down temporarily if the boiler temperature gets too high ( usually caused by being off level )
To avoid damaging the cooling unit . ( Personally have one on my fridge and they work well)
I installed their version which also has a thermostatic fan control and condenser fan to help cool the external fins in hot weather
https://www.arprv.com/purchase.php#number-threeI've not had any experience with the dual cooling circuit units from JC refrigeration but I have install several of their other units and had nothing but good service from them I would not hesitate to use any one of their products.
I added an internal fridge fan unit to the fins in my fridge several years ago which resulted in much better and more even cooling in the fridge
The best benefit was not only faster cooling but the fridge fins used to ice up requiring defrosting in about a month and a half now I can go for 6 months or more with not ice buildup.
https://www.amazon.com/dp/B08YJ6TLLR?RU=%2Fgp%2Fproduct%2FB072BPYXS8Hope this helps
Eric