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Question: 1992 Firebird won't start/stay running

I have a 1992 Pontiac Firebird V6 3.1L. I bought it last year from a guy that got it from another guy, and it had been sitting in that person's garage for about 9 years. Recently, it started acting up; a few weeks ago when I put some gas in it, the car wouldn't start up. I had to put my foot to the floor and it started up and drove fine afterward.

A few days ago, it did the same thing; I had to put my foot to the floor and it started up and ran fine afterward. The day after that, my dad noticed that the EGR valve wasn't on completely, and apparently the screws had rotted out and the whole valve was pretty bad. We replaced the EGR valve, and afterward the car ran fantastic and was much quieter than it had been the entire year I've had it. We then decided to go ahead and do some other stuff with the car, such as check the spark plugs and replace the fuel filter.

Took it to AutoZone, bought new spark plugs to be on the safe side (the ones in there now appeared alright, but I figured that I might as well replace them) and a fuel filter. I was getting low on gas, so I took it to the station and put gas in. Afterward, the car wouldn't start. I had to put my foot to the floor to get it to start, but it wouldn't stay running for more than 5 or so seconds. Eventually I got it running, pulled out on the road and it immediately died. This happened numerous times until I finally got it out of the parking lot and down the road, and then it did it twice more before getting home.

We replaced the fuel filter once I got home, and the car started up immediately on the first try, but afterward it started doing the same thing (needing to put my foot to the floor and dying a few seconds later).

I can hear the fuel pump humming when the key is on, so I know that it's at least working to some extent. My father also checked the pressure in the fuel lines, which was almost 50 PSI with the key on, and then around 43 PSI when the car was idling (it started up on the first try, but just like after changing the fuel filter, it started doing the same thing again afterward). Also, it wasn't registering any pressure after shutting it off and turning the key on again, but it continued to show 43 PSI when the engine was running.