As mentioned it sounds like either turbo oil seals or valve stem seals to me. The pulling up to a set of traffic lights and a puff of smoke coming out the exhaust is a classic sign of that.
Although you are saying white smoke, which usually means water as opposed to oil, when people say blue smoke, it just looks like regular smoke with a slight blue hue to it.
Have the 19 year old turbos on the car been changed? If not I'd start saving for some now, they don't give a lot of notice before they let go in a spectacular fashion, leaving you with a stub where the exhaust turbine was.