Problem with Hotpoint Mistral FF92P not keeping temperature?

I have a Hotpoint Mistral FF92P
about 6 months ago I had a problem with it not keeping its temperature, I was instructed to switch it off and defrost it for a day. This worked and the fridge, freezer has been working fine until now.
I first of all defrosted it over night then switched it back on to fast freeze. The following morning I came down to find that the unit was still on fast freeze and that the freezer had now reached -28 degs but everything seemed to be ok, so I switched from fast freeze back to normal operation. Later that afternoon I went to put some goods back into the fridge only to find the temp in the fridge was up +12degs and the freezer up to -17degs (it normally runs at -22degs).
So I repeated the process of defrosting the unit but this time I left it 2 days. After 2 day I powered up the fridge and left it to cool its self at a normal pace and not use the fast freeze function. Again the fridge and freezer got down to temperature over night but again failed to hold its temperature.
I am a electronic and mechanical engineer so I have a basic understanding of the fridges operation and therefore I should be able to follow any technical instruction anyone can offer

Aaron
September 2006