It is not expensive, to do your self.
Bought part local. $60-90 I believe, going from memory, I am thinking it is the decel switch, cut have to check a few things.
Most of all be sure auto is in tune.
1. Go to computer on passenger side fire wall, remember how many clicks you go. Move knob all the way down or up and see if the engine responds to this. If so you computer is working.
2. Check the vacumn advance on distributor. Pull air line off on vacumn unit attached to dist. If engine responds to this then changes are your advance is working.
3. These two parts working, then switch decel switch beside the throttle body intake horn on manifold. You will then have to adjust your computer, maybe. Leave at setting that was hopefully made when decel switch was working.
This solved my problem quite well.
Best with your new auto. I am repairing and upgrading mine myself, learning all kinds of stuff about these vehicles.
Take a look at these products, have done excellent for my 72 SLC, I was getting 13-14 around town and I am now getting 16-17.