Need input from big cam users.

Back in 1971 I installed a Stewart Warner vacuum gauge in a new 1971 Roadrunner with a stock 383 and it always bounced around at idle. It could be in my case and in your case the vacuum gauge it just too sensitive or the port that it reads from is not a stable source. I can't remember what source my Roadrunner used but my best guess is that it was just simple manifold vacuum. I have read in the past that some vacuum gauges require damping in order to stabilize them.