Why am I paying (again!) and underpayment penalty. If turbotax is correctly calculating the tax I owe, then I pay that amount, why is this showing up every year?????

If you owe additional taxes each year, that means you are not paying enough taxes in during the year. This is quite common and the easiest way to fix this so you do not have this issue is to adjust your withholdings on your paycheck. You can have more taken out from each check and then at the end of the year, you should not get the underpayment penalty anymore.