SMEG

Model s 28 strp/1 american type fridge with icebox.Fridge temperature seems to respond to stat setting but to get icebox temp down below -18 the stat needs to be set at number 4.At number 3 the icebox looses temp,however the fridge still runs around 6 degrees.To get the icebox to run cool enough the fridge needs to run at around 0 degrees.However icebox reaches -4 at times and down to -33,at fridge temp range 0 to 8 degrees.
Long description but difficult to describe,would it help if icebox was full,or stat problem,the time the stat is off causes icebox temp to rise out of range while fridge stays in range.
Please help