w126 High RPM with low speed
 

When my w126 300SE runs on highway it fail to achieve the normal speed of 110/120km/hour. It can only achieve a speed of 60km/hr and the rpm is as high as 4500-5000rpm with a very rough engine noise. Only sometimes the speed will go back to110km/hr for a short distance then it drops to 60km/hr . At times it comes with a jerk of about 5-10 minutes. My friends suggest to replace the existing old kick down cable as my car does not have a kick down feel. Can you please help? Thank you.