We were scheduled to fly at night, west coast USA to Europe when the plane came in with this complaint: "during acceleration after takeoff a loud scraping noise in the nose was heard, followed by failure of both weather radar systems." Checking the book, the same complaint had occurred on each of the previous three legs, one being at night over the entire length of Africa. As a fix, the antenna had been replaced at our home base, but to no avail. We asked the local contract engineer what he was planning to do and received the reply that he was going to replace the antenna. I tried to politely ask why that would help, considering that this had already been done twelve hours previously, but he brusquely said "I got a telex from your home base telling me to replace the antenna, so that's what I'm going to do." Anyway, we tested the radar during taxi out, it worked fine, so we took off, and sure enough, at about three hundred knots the whole system literally ground to a stop. Now it was winter, so we weren't expecting to need the radar much, but at this point I thought, enough is enough, so I convinced the captain and other co-pilot that this was not the way professional airlines operate (weather radar is required at night acc the MEL), and we dumped tons and tons of fuel and returned to our departure point. On the ground they opened the radome and even I saw what the problem was - there was a big greasy stripe across the insulation layer where it had been coming loose under the increased dynamic pressure of high airspeed and trapping the antenna so it couldn't move. Four hours later, with a new white nose we again left for home, tired but glad to have stood up to the "get the airplane moving again at all costs " mentality.