Engine shakes at idle?

Car shakes when I stop at stop light while break is applied and in gear. Shake is louder when ac is on. When I turn ac off it used to go away, but now it get less but does not go away. It is also louder in the morning. It gets little louder when I turn the wheel a little to the right and it keeps shaking when I keep the wheel at that position.
rpm at idle is at 500. Every now and then it goes under 500 when I am stopping at stop signs and stuff ( happens rarely). I had bmw to change the valve cover gaskets 3 times cause they could not get it done right at first. I thought this might have been because of the old spark plugs. I was planning on changing the plugs, but coil #4 failed before. So I changed the coil and the plugs at the same time. When I pulled the plug #8, bottom part( where it sparks) was covered with oil, but none on top part that coil connector plugs in. Took it to bmw again and asked them if they might have done something while changing the gasket, but I was told that valve gasket was good. 3 days later. I got a engine code p0300 which is random misfire. So I took it back to bmw again and asked them to see if there were still oil on the plug and if it causes a short. They came back and told me it was #8 coil needed to be changed. I changed the coil as well, but the shaking and the all I mentioned above still there. Would that be because there is a vacuum leak or vanos seal or fuel filter or exhaust or engine mounts? I just cant think of anything that I can go to dealer and ask them to look for. I know if I take it to bmw they will want to change so many things and none of them will fix the problem unless I change everything possible.
thanks a lot.