At any steady engine speed much above idle, the distributor housing should move several degrees counterclockwise, and stay there until the speed drops again. It sounds like you have a leaky advance diaphragm. The old school method of checking a vacuum advance was to try to blow cigarette smoke into the tube and see if it came out of the housing, but trying to find a smoker to assist with the test is getting difficult these days. Try removing the tube at the distributor and attaching a vacuum gauge to it. There should be no vacuum reading at idle, and almost the same vacuum reading as can be measured at the port you plugged with the engine speed at a steady fast idle (1,000 RPM or more).
