Our 2001 monte ss (3.8) has a vacuum hose on the back of the engine that when unhooked the car idles higher, and the vent actuators for heater/ac don't work. However once we plug it back in 3 out of 4 attempts to start the car unless I give it some gas and when it does start without assistance it will rev up and down between quitting and 2500 rpm before eventually settling at 700ish while in park. Then randomly if it sits in park it may act up and maybe quit again, however if I put it into reverse or drive it it will be fine for that duration until I start it again. Heater and AC vents work during this time until we unhooked the hose again btw.

 

The Vacuum lines will cause issues for sure, maybe it is not plug into the proper source. I had one once on my sons car that he told me it wouldn't start. The vac line wasn't attached at the firewall area must of fell off. So getting it to start was not easy had to keep your foot in it. Once reattached it functioned properly. Similar issue with an SUV that had a 3400 in it. Once i found the the leaking hose and fixed it all was good.
you possibly have a hose that maybe is bad still leaking or is hooked up to the wrong spot?

 

The thin, hard, black plastic line goes under the cowl, down between the fuse box and the battery, then through the firewall. It then travels to a ball-shaped vacuum canister under the AC control cluster. A small green vacuum line then goes to the AC position selector actuator(feet, face, or defrost). I suspect a large vacuum leak on your engine. I would first check the large black vacuum line that goes from the top of the engine to the brake booster. From there try spraying carb cleaner in short spurts as the engine is running.