It sounds like you either have a leak in the sealed system, or a partial restriction. Try this: Find a place to keep your perishable food. Unplug the unit for at least 24 hours (48 is better). Plug it back in. This doesn't work 100% of the time; but if there is a restriction, sometimes doing this will clear it up.
A leak can either be very gradual, therby giving unnoticable symptoms for a long time, or it can start small and increase. Depending on where in the system it develops, repair can be impractical.
If the trick works, be prepared for the same thing to happen again. What it means is that there is something (moisture, debris, etc.) besides refrigerant, coursing through some very tiny passages within the system.