Engine stumbling/stalling in drive

I have an 83 c10 with a new 350sbc. It’s been running like a champ for the last 3 months. As I had it out yesterday I noticed it started to stumble after letting up on the accelerator and applying the brake coming up to a light and it progressed got worse to where it would die. The engine idle in park is at 950 and after dropping it into drive it will drop to around 650. It’s never done this before and I haven’t changed anything. Thought it might be a bad ground as it was hesitating when starting. Worked on all grounds and now it starts just fine but it still stumbles after putting it in drive. I haven’t changed anything and now it’s dying. Can vacuum leaks just start all of the sudden. I also have an Edelbrock AVS 2.

idle at 950 is high i do believe. Mine idles around 500. Have you checked timing?