All the vac lines are good, I replaced any subpar ones in the spring. If this helps, I took the car for a spin around the block today and after driving for a few minutes it ran normally so it only happens when the car is cold. I let it run at fast idle for about a minute before dropping it, once I drop it the idle gets so low it sounds like it's gonna stall......
How much is it shaking when it's ready to stall? If the car shakes a lot then that means some pistons aren't firing properly and it's an issue with fuel/air/spark mix. Otherwise, it's likely it's getting either too much or too little fuel, or due to a cold engine, not enough ignition.
Additionally, the 307 gets its heat data from a coolant temperature sensor as well as the MAP sensor; Manifold vacuum increases due to the engine heat as the car warms up and adjusts several things within the vacuum system. If both of these means of collecting data are absent, the OBC will allow the engine to warm up for 30 seconds before allowing the high idle cam to be dropped when you press on the pedal and the engine will be so cold it'll stall. Otherwise, it high-idles until you step on the pedal. You can troubleshoot that one by allowing it to warm up for 5 minutes then driving it around and seeing if it's heat-related.
The other thing you want to check is to see if you have negative or positive vacuum on the ILC (idle lever control). Some vacuum system configurations, like on my car before I fixed it, will put positive pressure into the ILC, causing the screw to draw back into the ILC and choke the air intake too much. It needs negative pressure from the manifold (meaning, it needs the manifold to suck air out of it to push the screw foward). This can be checked by taking the air snorkel off and observing the two small barrels on the carb. As the car warms up, they should slowly open completly and when you kick it into low idle they should be over halfway open.
I tried again today and I noticed that when I dropped the fast idle, it did it for no more than a couple seconds, then fixed itself. It was significantly warmer today than the other days I did this (close to 70*) so something sticks when it's cold and doesn't get it's full movement. Possibly the ILC like Gwokable mentioned. I've confirmed now that it's got to be heat related. So in summary, it just needs more warm up time? I usually let it run for about a minute before dropping it.