Jeep 1997 Code P1494. My car will not go over 30 mph uphill and dies 3-4 times a day. And i'm broke! HELP?

I have about 9 codes. most involving cylinder misfires. but the last code showing up is p1494. i have read this is a dealer code, having to do w/ the leak detection pump. my car audibly sounds like it is leaking air beneath it, barely goes over 30-35 miles an hr uphill, and is eating up my gas at nearly 1/2 the mileage it used to get. when i open the gas cap, it hisses for a few seconds every time. it seems like there could be a vacuum issue, as I am not getting much power. multiple times a day, the car will turn off completely, at a light or slow turn, or even in the middle of the freeway. it 'puts' off, sputtering out slowly. sometimes, it used to do this for a a few seconds, then kick back on and drive normal. but slowly but surely, it has become more and more common, and is now just the permanent state of my car. this car has many other issues. it has 3 different sized tires. 2 of the tires are bald. the differential fluid is black, and needs to be changed desperately. there are various electrical issues, and some mechanics have said a sensor is bad? I bought this car used for about $1300. I had a new catalytic converter put in, had the oil changed one paycheck, and the spark plugs changed another. I've been doing things little by little as I can. I know there are many things the car needs, but in this economy, i unfortunatley have to prioritize. as in, which one will get me to work this week, so i can even get my paycheck. lol. i would much prefer to fix this issue myself, even if it does involve more of my time. at least i will learn about my car :) Can I fix this myself? What exactly is wrong w/ my car? I appreciate any help.