Service engine light on in my 2000 cadillac deville!!?

Hi i drive a 2000 cadillac deville and my service engine light is on..and for the past month or two the engine starts to shake violently while i am stopped. i took it to many mechanics and they cant find what is wrong. I replaced the filters,spark plugs, a/c hose and some sensors. But i cant figure out what is wrong with it...and sometimes the light flickers for about 7 times then turns solid again. And when i was changing the spark plugs there was some oil on them what might this be also? can anybody please help me so i can get this fixed!! thank you!