Recently, it somehow got an air bubble inside the lines in the engine. It randomly started overheating, but then would almost immediately cool back down once turned off. We never allowed it to completely overheat, had it towed and that's when they found and vaccuumed out the air bubble. Ran fine for another 1000 miles or so.
Now, it's starting to act like it's getting another bubble, though the symptoms aren't as severe as before. It has randomly overheated once more, but cooled back down and then was able to drive the 10 miles to get home. On the way home from that incident, it started to overheat again, but cooled itself back down without being pulled over (probably 30 seconds of temperature rising, then it fell again).
It's vibrating like crazy, having occassional trouble idling smoothly, and seems unable to go over 40mph. The heater at one point started blowing cold air, but now it works again without us having done anything.
Any ideas? If it's another air bubble, any clue why it's getting air in there?
Thanks very much for any help!