Hello mate,
The electronic steering on the punto can turn into an expensive fix when it starts going wrong. In your case though, if its only coming on if the car has been left for a few days and then going out untill the car is left for a few days again, it would suggest that it could possibly be to do with the condition of your battery. The electronic steering can be sensitive to when your battery is on the way out and if it does not get the correct voltages etc then it does not work propely. I would check the condition of your battery and the connections before i start worrying abou expensive power steering ECU replacement.
Hope that helps,
Alan