Code Reading: "12 21"

I believe the 12 means a code is about to be shown. I have a 94 Dodge Caravan 3.3L I believe its the sports edition, I have several questions;

1.) What does check engine code "21" mean?

2.) Is it expensive to repair this issue? (Im a student and have a very tight budget.)

- Here is what I have witnessed. Whenever I drive on the highway my engine hesitates and does a cardiac arrest motion and starts jolting.

- As soon as I get off the highway it loses power and I have trouble gaining full power again.

3.) Does this problem have anything to do with my fuel supply? My gas gauge likes fluctuating on its own one min im full next min im half tank next min im empty. I avoid this issue by filling up 10$ everyday when I can. Can you suggest anything that can help? Someone suggested it was my floating device in my gas tank but I think its bigger then that. I tend to smell gas like its leeking or something but I never see anything. I have a feeling its my fuel injection because it might be related to my check engine code "21" problem PLEASE any ADVICE WILL BE GREATLY APPRECIATED! Please email me aswell if you can Im not posting my email but I believe adminstrators of this website might have it.