The more we follow this thread the more I think we have a software problem. The fact that Sandra didn't find anything wrong with the hardware strengthen this feeling. I still do not exclude a hardware problem but I think that we investsed considerable efforts in this direction with no results.
Had the computer been with me I would have formatted it. As a matter of policy G2G do not recommend reformatting. Accepting this policy I suggest other correcting steps short of clear format.