Is it oily or is there a considerable amount of build-up? You have to remove the intake manifold to clean it. I'd advise that you take it to a machine shop to get it cleaned because it takes some nasty cleaners to remove the build-up. I haven't heard of anyone who noticed a performance improvement from cleaning the intake manifold.

One hypothesis on the rod bending problem of the 3.5 is that chunks of build-up get sucked into the combustion chamber.

Sixto
95 S420
87 300SDL