Starting Problem and Poor Fuel Economy

Yeah I know, I still have this car...

These problems I'm about to mention may or may not be related - I'm not sure.

On Friday, I stopped at the gas station to pick up a few things. (Not to get that expensive gas.) Then, the car did not want to restart. The turning over had no pattern, but eventually the car started. It then ran for a few seconds, then died. It then did this again when I tried to restart it. It did this several times. (Approximately 10) The car would run as long as I had my foot on the gas but if I let off the gas, it would die. I finally said forget it and just lightly pushed on the gas and put the car in reverse while keeping my foot on the gas. I then let off the gas and it was fine from there on out. Now the car has done this starting problem before but this is the first time that I've had to keep my foot on the gas to keep it running. Yes, I'd have to keep my foot on the gas for it to start but not once running. To me, it sounds like the car's starving for fuel. So much for 'fuel for the soul'. Now everytime that this problem has occured, my gas gauge is at or below 3/4 of a tank. (I think this is actually half because I always seem to put in 7 or 8 gallons when my gauge reads '3/4'.) I rarely let my car get too much below '3/4' before filling up. It's never done it to me when it's full. Is it the fuel pump? If so, I'll be very disappointed because it needed a fuel pump when my brother bought it with only 60,000 miles and was replaced and this means that it kills fuel pumps every 60,000 miles. How much would it cost (approximately) to do the fuel pump with labor if this is what it is and is my fuel pump on the Pontiac 6000 located in my 15.8 gallon gas tank? My dad is getting a little older and probably wouldn't want to deal with replacing the fuel pump anymore. (I believe he did the first one, and it was a mess if I recall correctly as I was still a child then.) Now the other problem was 3 or 4 weeks ago, I took the car out on an hour and 15-minute trip. Out on the highway, I only got 15mpg where I used to get 30mpg or 31mpg even in the wind going to the same place. What gives? Why so low? Didn't drive any differently than previous times. The car 'seemed' to be running fine. Checked fluid levels and tire pressure before I went. No extra weight. The only difference was I had my windows rolled down, but I don't see where it would have cut my gas mileage in half. Do you think by chance that this is related to my starting problem since they both do with gas?

Any clue what's going on? Thanks.

P.S. - The bracket for my muffler fell off. It appeared to be welded on but does it have to be welded on?