Try lubricating the throttle linkage first, but it is very likely the cruise control amp. Some people have successfully resoldered bad connections on the circuit board. Also, there are reputable companies that rebuild them. Removing the old one and installing the new one is a simple DIY project--search the archives for instructions on your model year. Cheers, Andrew Seidel.
85 500SEC Lorinser
83 300D Turbo