My 1991 Cavalier (2.2L with non-functioning a/c if it matters) creates some smoke/steam/vapour sometimes - but not all the time - when I am stopped at a light or stopsign. There is quite a lot of it. It comes up from between the engine and the radiator. The (I think) exhaust manifold (the piece bolted on to the engine and then goes down behind the engine from the front of it) is now largely white in colour. The problem has been happening for 2 weeks just after my wife had the oil changed. The oil change guy said antifreeze was needed so he added some. Once I noticed the problem I checked the oil and noticed that there is too much oil by about 1/3 inch above the max line on the dipstick. I didnt think this was a big deal so I ignored it (maybe it is?). Then I drove 350 miles and when I was about to do my return trip I checked the fluids and noticed that the oil was the same and the antifreeze jug was empty completely which was odd since (or should I say if) the oil change guy added some. I topped it up appropriately along with some stop-leak (I assumed it must have leaked out) and figured that was the problem. When I park the car for a long time there is no antifreeze that leaks out.

Does anyone know what the problem is?

Have you checked the clamps on your rad hoses? If the antifreeze is leaking during operation then most likely you have a loose clamp somewhere's that the water pump pressure is bypassing. That or you have a small hole in one of the hoses that is doing the same thing. I wouldn't think it would be a hole in the radiator itself if you've added stop leak and it's still leaking unless it were a rather large hole, in which case it would leak even with the engine off.