I know exactly what the problem is. If the RPM's are high or extremely low all the time, you would have to replace the COMPUTER and IDLE CONTROL. My friend had just done this job on his 560 and it cost hime around $500-$600 for a replacement and labor. But you can also get a used one from the junk yard. This is a common problem with all W126 V8's but it is nothing anyone w/ a w126 should worry about unless the RPM's are really high or really low all the time.