In addition to the air in the line, what does the inside of the radiator itself look like? I had a similar problem with our Jeep. In addition to a thermostat that was intermittantly freezing up, it was discovered that the passages inside the radiator itself were really gunked up (yes, that is a techical term ) with mineral diposits and such. They have a way of breaking someof that stuff loose with small rods.
If your radiator is in good shape, and deposits are the cause it may be fixable. In our old Jeep the radiator was starting to rust away and puting rods in it would have been the end of the radiator...we had it replaced.
"In the long history of the world, only a few generations have been granted the role of defending freedom in its hour of maximum danger. I do not shrink from the responsibility...I welcome it!" - JFK