First I have a 57, Ford 312 , 57 dizzy , Holley 570 CMF carb. New plugs, was running good. Started to notice that under rpm around 3500 plus rpm. The tach started showing some goofy behavior.. would pull steady till the 3500* range motor stared a slight surging, tach. Show crazy needle movement, but rpm sounds some what steady.
When trying to find out what was happening. Check vacuum it showed 18 psi, timing 9 deg .
Look thing over but do not know what it might be. As before I need you great advice as to what going on and where, and what to do to fix issues.