cooling system stops working

After running for eight year with no problem, the cooling system stopped working one day, with only the light in refrigerator on. I opened the back panel and vacuumed out all the dust, and then opened the door and pushed the temperature sensor lightly. It worked for a week, then stopped again. I did the same thing as before and put some oil on the fan. It worked for 2 more weeks and stopped. This time I repeated what I did the last time and it worked again so far. But for how long, who knows? Can anyone tell what the problem could be so that I can fix the problem for good?