cause of white smoke?

[agazmi]

Hi, i have a 1990 300E with the 3.0 straight 6 sohc M103. The car has about 109500 miles on it so far. Since i bought this car it has always had a puff of white smoke pouring out of the tail pipe, with that said it has passed emissions check Twice with very good numbers. when i first got it it had issues with idling to the point where it almost dies out on me in a stop light, i used two bottles of techron fuel system cleaner so it no longer has this problem, but the white smoke continues. Once my dad had an oil change using synthetic oil and the oil slowly seeped out of it somewhere, so i thought that it might have been burning the oil through valve guides or valve stem seals but when i put in regular oil (to spec) i have not had any oil loss. with that said the fairly new spark plugs (platinum or some other) are covered in carbon (completely black). I also feel that the car is a bit held back when it comes to acceleration (i got beat by a neon lol...). gas mileage is terrible i barely get 10-15 mpg. there is also slimy black stuff in the exhaust (probably carbon?). I did not note any loss of coolant or mixing of coolant in oil or vice versa so no head gasket issues. Another thing i noticed is that the transmission box will some time clunk (like a piece of metal keeps hitting a rotating object) when i first shift into drive or reverse but will stop after the car starts picking up speed, it will also make that noise when turning left or right at 15-20 mph.

sorry for the long read but, i honestly cannot pin point the problem(s) and i really dont want to start throwing new parts at it. What i have come to as a conclusion is that it might be the following parts hopefully you guys can help me weed some out->

-O-2 sensor
-catalytic converter
-air/fuel mixture ratio
-ICV sensor
-Fuel Injectors
-Vacuum leak
-piston rings?

Parts that have been changed so far
-Throttle body
-air & oil filter
-tires

I would really like to have this car in perfect running condition, As a mechanical engineering major (2nd year) i dont mind rebuilding the whole engine as a project with fellow ASE Mechanical Engineers in my college.

Thanks in advance for your reply!