I had that on my lumpy 440 cam install

I had to add a lot of initial timing and recurve the dist with the FBO mech advance limiter plate.

I now have it locked on 0* and it is set at 36* total runs like a beast and idles 850 rpm

check to make sure the carb throttle plates are not open too far and on transition slots.

I chased a idle problem for a long time. get it idle good but when stopped on street in traffic it would be idled up a few 100 rpm and I would have to blip the throttle a couple times to get it to drop to curb idle setting.

vac pod leaked for 1 sluggish on acceleration with no vac advancing and weights/springs were dry gummed up not letting it drop back at idle.

get a new reman dist and ditch the high mileage unit.