I couldn’t follow your post. Low RPMs, sometimes it starts? Sometimes it goes to high rpms and then lowers to 3000 to 4000 RPMs? Then offs?
Maybe you mean sometimes when you start it, the engine go up to a higher rpm, and then will drop to a low engine speed like 300-400 RPMs and sometimes sounds like it is going to die, but continues to run? 86 had fuel injection, I would look at the idle control valve which likely could be malfunctioning. It’s an easy component to change out. You can even just tap on it when it is idling low and see if the RPMs climb. If it does, it’s hanging up. You can try to clean it, but usually requires replacement.
First Edition 4dr
1967 Bronco
1993 Mustang Notchback