I assume you meant 1000 - 2000 rpm idle, not 10 - 20 rpm. In any case, high idle issues on our cars (mine is a 1988 560SL) are most often isolated to vacuum leaks. How is your "economy" gauge reading? It should be pegged hard left at idle. If not, that indicates a massive vac leak. Start looking and listening. Use a mechanics stethoscope, a very inexpensive and useful tool.

There is a common vac fitting often overlooked between the rear of the intake manifold and the firewall. It is hard to see and reach and is usually the first to go since it is in an environment of oil vapor and high temps.

Good luck and please report your findings and repair to add to the data base.