Quote;
"...whitish looking bubbles..."
Damn bro, I hope you don't have a cracked head. With you saying that you've been back to the dealer multiple times for repeat coolant caps, level being low and whitish bubbles might mean just that. The cracked head will start allowing exhaust into the coolant system and raise the temp to include over pressurizing the system (exhaust gas) to the point of it venting out, loosing coolant. The dealerships mechanic should have caught this, since they haven't figured it out yet I'd be questioning their troubleshooting ability and asking for some money back because of incompetence.
Good luck, hope your car is still under warranty. Maybe thats why your dealer doesn't what you back.
P.S. Go to this website and read these posts. Do any of them sound like your symptom's?
http://www.2carpros.com/topics/whitesmk.htm