Ok i have had this problem with a 80 450sl before, it was the pre cats in the exhaust pipes just below the exhaust manafolds, you will have to take the pipes off the engine and see if you can blow air through them ,if they are bad like mine was they will look like someone has poked steel wool in the pipe the ones on my car were stoped up about 90% and the car would not go over 50mph, and was getting HOT!, there is another cat on furter down in the exhaust system ,that might be stoped up as well, good luck, hope this might help you....

85 500 SEL EURO (no cats on this one)
73 450 SE