After the hint of our electrician both old and newer PCs(Motherboards all fully open) are now "perfectly" earthed. I prepaired the plug in the way that only the center plug is active. the other two holes are filled up-cannot takeup contact for currency.
Maybe i should tryout now an touching or move MOBO to find out if pc will crash?
line goes to old PC. Old connected with new PC with cable. What do you think if that? Most cables which caused troubles on my feet-space are out of the way. Only printercable there.