Problem Fixed!
A friend of a friend who tinkers with old Rolliflex cameras came by to
look at it. Since I had planned on taking the camera in for repair, I
figured there was no harm letting him handle it. He gently held the
mirror down and went through 4 winder/shutter release cycles. That was
it - cleared the problem and I've since wound/released the shutter
about 100 times with no problem. He said that he'd seen similar
situations before on other SLR's and it had to do with a faulty/dirty
mirror lock-up mechanism.
Got lucky this time...
Bill.