White Smoke

[bustingear]

I know a few posts have been written on this but I am getting white smoke on my 99 Boxster with 151,000 mikes. I have replaced the AOS so I dont think that is it. The car has gotten worse since the weather got cold. It was not bad in the summer. Now especially after it sits overnight I get white crop duster type smoke until after I have driven about 2 miles. It has been like this about two months. Then it goes away. This week I went on a business trip and came back and it is acting a little worse. The more it runs the better it is but still smoking white and running rough a bit when started cold. It still goes away after about a mile or two. I have added coolant about 4-5 times in the past 4 months. I dont see any froth under the oil cap. The coolant cap does take alot to turn and when it does come off you can feel the pressure wooshing out. I do smell anti freeze while driving and the windows cracked. I also smeel it faintly when I get close to the exhaust. Could this be the heat exchanger???? It is the least expensive to fix. How do I tell the difference between it and somthing worse like a cracked head, slipped sleeve or better a head gasket? Suggestions? Thanks for your review. Dennis