1992 Buick Lesabre Alot wrong here.

Engine Performance problem
1992 Buick Lesabre 6 cyl Front Wheel Drive Automatic 97k miles

It has a very low idle and shakes alot. When I put her in park she will die more often than not, same as switching to reverse. Also I don't know how many mpg a buick should get but mine gets 8, I know this for a fact because I tested it. I also know that a buick should get more than 8. I was wondering if you had any ideas as to what might be wrong, I was thinking the throttle body or the fuel pump but I would like a second opinion. Thanks in advance for any help.