Radiator issue

I have a 2000 Oldsmobile Silhouette. I brought it in to have it looked at because it keeps overheating. They looked at it and said they couldn't get it to stop bubbling. (Exact words). They said the anti-freeze was all going into the over-flow and getting there and not circulating through the radiator. Just a little over a year ago I had it in with these people and they replaced the intake manifold gasket. They drained and flushed the cooling system, inspected hoses and clamps, flushed and chemically cleaned the system and checked for leaks, there were none. They pressure tested the system also. Also, there is (not any steam or leaking) going on. OK, so I drive a very short distance maybe a mile 2 ways and I watch the needle go up and up until the overheated cooling temp comes on. And that is after shutting off the engine once. My question is, after all the work they did and with this still happening, what could be wrong?