Hi Joanna, I have a similar problem with my fridge having to be set on near
maximum cold setting to stay at a reasonable temperature. This has occurred
since new. I drilled holes in my cupboards to allow more air flow which
helped to  keep the temp down a little more.  I would say your problem could
be the thermostat, which arent very expensive and quite easy to fit. you get
a helpfull domestic shop and he mite point you in the right direction. SEANY