Whirlpool refrigerator not cooling properly

[Lim]

I have an old (possibly 1995) SxS Whirlpool fridge ED22DW. Just recently, it has been actiing up, so I cleaned out all the filth on the condenser coils underneath the fridge. It is cooling a bit better but it is either too cold or not cold enough. I am not able to use the thermostat on the front top panel to regulate. The compressor runs and it is a bit warm and the condenser fan works, I went behind the fridge and removed the back cardboard panel to check. When the compressor runs, the freezer goes below -10 degrees and the fridge part goes below 30 degrees. After a while, when the compressor stops,the freezer temp goes to 30 degrees and the fridge part goes to about 40 degrees. When I give the front panel (where the freezer and fridge controls are ) a sharp rap, the compressor comes on again and the temp goes back down to where it should be.
I am trying to save some money and not buy a new one. My husband wants to junk it and get a new one. Can this fridge be repaired? From the symptoms, can anyone help me diagnose the problem? If the thermostat is the problem, how difficult it it to replace it myself without calling the repair man? :

[RegUS_PatOff]

sounds like the Thermostat.

click on picture
http://www.repairclinic.com/dbImages/00000309/00094266.jpg (http://www.repairclinic.com/SmartSearch/SSPartDetail.aspx?PartID=2299)

Two screws and route the Temperature Tube into the same location.
(and 2 electrical connectors)

[Lim]

To: RegUS PatOff

Thanks for your reply.

I have 2 manual controls. One is for the freezer and the other is for the refrigerator.
Are these two compartments controlled by one thermostat?
Thanks for your reply.

[RegUS_PatOff]

The pictured Thermostat controls the Compressor.

The other control is mechanical that controls an air-flow damper