Need Help With Off-Idle Surge

I've been trying unsucessfully to tune this very slight surge out and am getting frustrated. It is a slight, repeating at a regular interval surge at what I'd call parade speed: Just rolling along with my foot barely resting on the accelerator, ~975-1100 rpm. Put your foot into it a little bit more and it behaves totally as it should.

My combo:
383 w/ 9-1 compression
Cam: Comp #21-670-4 "Nostalgia"
Dur. 280/287, Lift 474/474
727 stock rebuild
TC is TCI ~1800rpm stall
3.0 Rear gears
4000lb car

This combo ran fine without this problem till recently when I replaced my old dizzy (no vac advance) with a new Firecore w/vac advance, and replaced my old Edelbrock 1406 with a Street Demon 625 cfm.

I tried opening the idle mixture screws 1/2 turn (and backing the idle back down to 850) and that didn't help. Gave it a little more initial advance was at 18*, took it up to 20*, but that still didn't help.

What do I try next?