Laptop Power Goes Off Unexpectedly

I have an Acer Aspire 4738Z. It recently goes off, even when the battery is full or it is connected to the adapter. I never experienced it before. It never goes off always, I can use it without problems sometimes. It only annoys me when I need to do something important and the problem occurs. I have tested the battery and the adapter to a similar laptop. Both of them are working, long battery life, normal charging time. What seems to be the problem? The warranty expired last year. Is it worth it to have it repaired by a repair center? Or can I do something to fix the problem? What should I do?