Just out of curiousity what do the plugs look like ? I remember many years ago I had a 6 cylinder Maverick that did the same thing and everything I checked looked good. Then I pulled the plugs and they all had a good bit of carbon buildup on them. It would idle fine and run fine without the vacum advance hooked up and run bad when stepping on the gas with the vacum advance hooked up just like yours. I cleaned the plugs and it ran great Yea it seemed with all the carbon on them they would not handle the extra advance and would breakdown and miss. I know it sounds wierd as I really scratched my head on it but it is the truth. The plugs would not fire with all the advance when the vacum advance was hooked up and with all the carbon build up on the plugs. But like I said I cleaned or replaced the plugs (was a long time ago) and it ran good. Ron