Just my thoughts, but maybe your vac advance is starting to come in at or just prior to 800 RPM? Are you checking your timing with your vac advance disconnected? Have to ask cause sometimes people forget about that, even myself. Also, is your vac advance hose connected to manifold vacuum or higher up (ported) on the carb? I run manifold myself, but lots of folks have mixed opinions about that, my car just seems to run better with it. I would start by making sure to disconnect the vac advance hose to the dizzy and plugging the source and the hose, then put the rpms down to about 600 or 650, just to make sure that no advance is starting to come in and time it to what you need it to be. Then after timing is set, adjust your rpms to where you want it to be for park and in gear, then plug your vac advance back in and see where it goes from there. Here is a good article on Vac advance http://www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html
Here is one on this site https://firstgenfirebird.org/forums/ubbthreads.php?ubb=showflat&Number=230669 let us know how it all works out for you!