Does it rev higher in neutral and only to 3000 rpm under load? Are you sure it's not an ignition problem?

I'd supect the filter over the pump. I don't know if there is a strainer but if there is one it's in the oulet of the fuel tank. Look in the area above the right rear axle. A hose or two will extend from a large hex nut (~40mm). The strainer is on the other side of the hex nut. You have to drain the fuel tank before removing the strainer. Actually, removing the strainer will drain the fuel tank. It's just a matter of being ready for it.

Sixto
95 S420
87 300SDL