2002 Nissan Altima • 15,000 miles

5 months ago my check engine light came on and my car was shakey I was told to get a tune up and change my spark plugs as I did and I also got a oil change. My car only got worse the check engine light never went away my car stoled out while it was in idle I got it checked by 4 different mechanics and even a diagnosis code said I needed a catalytic converter but to avoid that they said it could be bad gas so I did a fuel injection cleaning 3 times. My car was losing oil I ended up getting a valve gasket and a used catalytic converter. My check engine light came back on after I drove 150 miles and my car still shakes and stoles out when in idle and it's still burning oil. What can be the problem?
May 3, 2013.

Start with a compression tests