Idle problem

The engine will be idling OK, then starts shaking very badly. Give it some gas and it idles ok again. When it is shaking the vacuum gauge is pretty steady. It's a 66 Cuda with 360. Vacuum advance is not connected. Comp 280H magnum cam, air gap with 3310 750 Holley. Vacuum is 10" in gear. MP electronic ignition with orange ECU.