86 Chev Celebrity stalling

I just recently bought a 1986 Chev Celebrity, 4 door, 2.5L TBI 4 cyl, automatic, with 110K from an older couple. They didn't drive it much, so it has sat a lot in its life. I think maybe all parts are original, (except for brakes) even the tires look 1986ish with the wide white sidewall ! When I brought it home, it started and ran good. It did seem to have a slight miss or engine stumble, but ran. I drove it to the stop sign, and it stalled as I was braking. It always seems to stall under braking when warmed and off fast idle. The transmission doesn't seem to downshift early enough, slowing the engine to stalling rpm. I found a loose sparkplug wire, and it seemed to idle better, but still stalled. When we removed the vacuum line from the sensor directly in front of the air breather, it idled a bit faster and doesn't stall. It does go into gear a little hard now. The garage said they checked the vacuum unit (that was unplugged) and said it was working as intended. The garage has already changed : o2 sensor, coolant sensor, and the vacuum unit on the outside of the air breather. Items also changed for safety : exhaust pipes, some brake lines, some fuel lines. I have put in a new air filter. I also got new sparkplugs, but are not installed yet. If anyone needs anything from the Haynes 38005 repair manual, it came with the car, just msg me. It covers Century, Celebrity, Ciera, Cutlass Cruiser(?), And Pontiac 6000 for years 1982 - 1996. Thanks for your time.