Engine overheated and died

The problem has been appearing for a while now, always when I was driving in a heavy traffic. The temperature gauge goes about half way up the scale and when I turn off the engine I can hear coolant fluid boiling in the reservoir. When driving on the highway everything works fine, but as soon as I have to move slowly or break or stop often this happens after about 30 minutes of 'city' driving. I'll also add that oil is burning as I have to add it pretty often when I drive a lot. Three times when the engine got hot it died when almost idle (when it tried to park and when I accelerated after coming to a complete stop). Each time I waited for about half an hour, added more oil (when I opened the oil cap, there was smoke coming from there) and the car started without much problems.

Recently, I was looking for a parking space in the city for almost an hour and when I found one and tried to reverse uphill, the engine died as soon as I tried to accelerate. The temp gauge was about half way up, but the coolant was boiling and the engine seemed really hot. This time after waiting a few hours the engine didn't start. I took it to the mechanic who opened one spark plug (the closest to the coolant reservoir) and a white smoke was coming from there (it was after almost a day since I last drove the car). He tried to start the engine and it worked out. I didn't do anything else, because I'd like to figure out what is going on before I spend more money on it. Anyway, the mechanic said that there were some gasses in there and that was the reason the car didn't start.

After I came back home I checked and noticed a little oil near the spark plugs. The oil stick comes out clean and the exhaust looks and smells normal.

What seems to be the problem and what can I do?

Thanks!