Car behaves bad when Low RPM and in gear coasting to stop

[hbrand]

Happens also to my Mercedes 280SL - same mechanical throttle setup as my
1982 Porsche 911SC

So I am coasting to a traffic light in lets say 3rd gear and RPM 1500. At one point the car behaves like I tap on the throttle up and down a bit. Car rocks back and forth.
To get out of that I need to press the clutch.

Could it be that the engine moves slightly and activates the throttle ?

Greetings, Harold

[smadsen]

My theory is, if you were coming to a stop in 5th gear, the car will start to "buck" as you near 0 mph. The "bucking" you feel is the clutch jumping around as the motor is forced to slow below about 1000 rpm or idle. The bucking becomes more violent the closer you get to 0 mph until finally the weight of the car bring the motor to a stop.

So, the taller the gear, the sooner the bucking starts as you slow down. Also, the engine speed can exacerbate the issue & make bucking onset occur sooner. In other words, you can momentarily stop the death rattle by slipping into 1st gear @ 1500 rpm as you bring the speed down with the brake.

It's important your idle is set correctly, prolly 900 rpm. I don't know what a gear chart says, but I bet 3rd gear at 1500 rpm is 35 mph. Your car may not be happy below that speed at 1500, so either clutch in at that point or bump it into neutral.