Transmission Gear "hunting"

[swilly43]

I have an '86 560SL with 72,000 miles that I bought last summer. The car has been well cared for and is trouble free. I have noticed that transmission will hunt for a gear when travelling 40-60 MPH and going slightly downhill. Tranmission selector in "Drive" and coasting. The tach will suddenly jump to a higher RPM during the downshift and then drop back down when it upshifts. This behavior will cycle in about 5-8 second period until the I get to the bottom. I do not think this will result in transmission damage unless it is goes on for a prolonged period and causes the tranny to heat up. Has anyone else experienced this? Does the Mercedes have a feature to try and keep the downhill speed in check? Crusie control is off. I could downshift manually but the manual discourages this.