Clouds of smoke on start up

[Najp24]

I've owned an '01 Boxster for 9 months now, and bought it with low miles but patchy service history. I have had the oil changed twice and fitted new ignition packs. I regularly monitor the oil level and there is no appreciable loss.

I am therefore confused more than concerned that, every so often, on start up, a cloud of smoke, obviously burnt oil, erupts from the exhaust. It lasts a few seconds and the engine runs as sweetly as ever. It doesn't happen on every start, and the engine has never sounded rough. There is no smoke at all afetr a few seconds.

Is this a typical problem, or a symptom of something sinister?

Thanks for any views!