trouble watching the image on the camera..

After a couple of days on the card, the camera can't read some pictures anymore... the question mark appear instead of the pictures... But when we plug the card in a computer we can watch the pics... any idea?

As mentioned in your other question, this is a common problem with digital cameras no matter the model. They are designed to write to a card better than they read. So it isn't uncommon that it may have trouble reading a sector one day and then read it another day. Or even one time it can't and then a few minutes later it can. It could be weather, subtle current issues or electric charges, even a bad sector on your card. I would recommend that you get your images off using a card reader and then format the card. In fact, formatting you card every 5 times or so is a good idea. But in reality, these cards don't last forever and do fail sooner or later. This could be a sign of that.