strange problem - need help

ive had this car for about a month and a half now. the whole time ive owned the car it has done a few things and theyre all related to each other. first i noticed that at about 2k rpm that is makes a very loud rattle type sound. i thought it was just how flowmasters sounded with no cats, but after the problem went away and came back i know different now. also when im goin 40 and shift into 5th the shifter, and the whole car for that matter, will shake real bad. whenever its doing this my idle hangs forever coming to a stop sign. how i figured out it wasnt supposed to do any of this is because i took a road trip recently and at just about the end of the trip all the problems had gone away. they stayed that way for the next day and then last night they were all back. i made another thread about the exhaust note but last night i figured out they were all related. sorry for the long post..but its like night and day and im trying to figure it out. thanks
 
well i found out what the problem was. but now theres another problem. my car sounded like it had a cam in it and would shake, so i started pulling the coil-on-plugs one by one until the cyclinder on the drivers side closest to the firewall made no difference in the idle. i pulled the coil out and it smelled really toasted. the plug itself had tons of oil all over itself and the threads. i put a new plug in (but a different brand from the others, is that ok?) along with an extra coil i had laying around and went to drive it. it was night and day. the car runs perfect now. no shaking in 5th at 40 anymore. i wonder what would make it randomly misfire like that without throwing any dtc's. also, do i need a new head gasket because of the oil on the plugs?