2007 Hyundai Accent White smoke continues

I have taken the 2007 Hyundai Accent for repairs four times in the past two weeks. The car has only 17000 in two years. White smoke comes from car at startup (not all the time) and when it gets up to operating temperature it sometimes blows white smoke on acceleration. The first time they fixed it, I was told it was a stuck thermostat. The second time I was told it was the car engine computer and replaced. The third time they told me they did a pressure check and adjustments but they can't find anything wrong but they made adjustments. I brought it home and after running some errands it blew white smoke at startup. They tell they cannot find anything wrong when they test drive it but when and I get it back and start using it, the problems start all over again. I am just so frustrated by all this. This is why I bought this commuter car new so I would have problems like this.