Logic Express 8

Since updating my iMac to 10.9.5, when I start the transport curser through the bars of a project, it drags blackness behind it which then hides everything in the arrange window behind it as it goes. Still plays OK but everything behind the moving bar line disappears behind blackness. It's still there when I stop the transport, but if I move finger side ways over the mouse to slide the bars forward and back the blackness has disappeared when I return to bar 1, but starting the transport again, and blackness forms behind the bar line as it goes forwards. Spooky!.. Anybody know what I might try..