1991 Pontiac 6000LE 3.1 Liter V6

I'm having trouble with my Pontiac 6000 LE with 130,000 miles on it. It is idling rough, rattling, shaking, and running like junk. It is also running very rich using nearly a half a tank of gas to go only 20 miles. It also stalls out, but will start again. It has also lost a lot of it's power, and only seems to barely chug along. Quite frightening when driving in rush hour with everyone nearly running you over because you can't get going fast enough.

It has been hooked up to a diagnostic machine, but the only thing given was a bunch of error codes that couldn't be read. It also kept dying out when the diagnostic machine was hooked up to it, but would start again once it was taken off. I have been told that the computer is probably the culprit, but I have also been told it could be a bad fuel pump, or a clogged fuel filter. Can anyone help me as to what is going on with this car? It would be greatly appreciated.