Need help, weird vibration, very frustrated

I am very frustrated, and need some ideas. I have a 2008 Malibu 2LT with a 4-cylinder. It has all of sudden developed a weird vibration that comes and goes, and it isn't there all the time. I have a 25 minute commute and it starts about 15 to minutes into the commute on the highway. It starts midly and then gets progressivly worse. I feel it int he front of the car, through the stering wheel, and as it get worse, when I step on the brake pedal. But after I have used my brakes a few times, or the car sets for a few hours it is gone, sometimes it comes back, and sometimes it dosen't. I have had it to the dealership here on two different occassions, and the second time it was there for 3 days, and it never accted up for them when they drove it. It feels to me liek it is coming from the left front, and after it acts up and I stop, it feels like the left front wheel area is much hotter then the right front. So I thought maybe a sticking caliper, but they said they checked it it everything is fine. But everthing is not fine, and I am begining to get very frustrated. I have never had a single issue with this car till now, and I am more frustrated with the dealership, I am not sure they are doing enough, anybody work in service have any ideas? Like I said it comes after about 15 to minutes of highway driving, and I feel it in the sterring wheel and the brake pedal when I step on the pedal, and it always goes away after I use the brakes a few times or he car sits for a while. And the left front feels hotter then the right front after it acts up, I havent noticed any excessive brake dust or burn marks on the rotors. HELP!