Electrical problem with my PRS-T3

I had a serious problem with my Sony PRS-T3. The screen is completely frozen and I have to open the device to reset it. I have check the board (visually inspection) and there is no trace of heating or water condensation).

And guess what, the device work perfectly and flawlessly until I put the plastic cover on it. And again it freeze and I have to go back to square one. I have also try using some spacer before closing the device without any success.

Do you have any idea of the localization of the problem. May be a bad connector or something like that.

I need your precious help !