I had been having the same problem with my 85 Jetta and the small return line outlet from the radiator was plugged. I actually had to use a drill bit in a set of vice grips to unblock it. A piece of haywire just wouldn't cut it. Some of them have a check valve in that line that malfunctions. Most people just take it out. Unfortunately, without flushing out the rad really well the crud will probably plug it up again (Ask me how I know :oops: ) Mine was plugged solid again when I checked it a couple days ago after blowing the heater core this weekend in a major snowstorm on the busiest street in the whole city :shock: .
it was definetly the cap i stuck the blue one on it off of my rabbit and the problem was solved, i thought it was my head gasket, thank god, now i can beat the *** out of my car more haha. oh yeah and i found an extra one in the garage, so no worries.