Lost connection to VAC server

[bbson john]

I have been playing TF2. My gameplay gets constantly interrupted by disconnection from the server, with a message box saying, "Lost connection to VAC server". Then, I find myself theoretically disconnected from Steam. My friends list said I was disconnected as well as the achievements list did not show up. I was so sure that I was in connection with Steam since I could rejoin other servers swiftly. What is annoying is that I lost my achievements in the previous game.

p.s. Upon one disconnection, I lost all my achievements. For example, I achieved "Head of the Classes", but now it shows up as "Head of the Classes 9/9", without a sign of completed.