(G) power failure?

I own a Punto active 8V (2005). This is my problem: A few times my car had a power failure without any specific reason for example: I`m driving the car normally and then I stop the car and turn down the motor and lock the doors. After one or two hours when I try to open the door I noticed that my central lock doesn't work (it is out of power) so I M oppening the door one by one, and the clock on the dashboard is blinking 0:00 like it has been cut out from the batterie. When I turned the key to give a contact all the light symbols on the board display doesn`t work like there is no power at ALL. Then I turn back the key and after a minute or two I try again and suddenly everything is back to normal. After maybe a few days or even weeks of normal work the problem is back again.Ther are some strange things happening (strange to the authorized service) like during a drive the back wind shield is starting to work without it has been turned on, or the central lock is going on and off during a drive all by itself. The problem repeats all the time but it doesn`t stays long enough so the people in the service can`t solve it(because until I bring the car in the service the problem is gone and everything is back in normal). Please help me with some ideas and suggestions!

Is it not under warranty?

Check battery terminals (disconnect battery to reset body computer) ...could be bad ignition switch aswell.

I have tried with the battery terminals but they are tight enough. Can you explain me what kind of part is the "ignition switch" and what is its role.{(I have tried to provoke the same effect(by touching the battery terminals like there is a bad contact) but nothing happened.}