Waeco odd performance

We have a 50 lt Waeco which has bewen great for about 18 months but now when we are using it as a drinks fridge while its out of the Prado and the lights which indicate cooling level keep dropping from the setting of 3 (preferred) to 1. When it gets reset sometimes they go up to 4,5, or 6. This is when it is on the 240 volt adaptor. I can't get it into the car to try on 12volt at the moment but I reckon it will still have the same problems. Any suggestions (nice ones)???