Loses power when you accelerate. Dies.

I just recently purchases a 1990 Mustang 2.3l LX from a friend of mine. He said it was having power problems, which I agreed with once I looked at it. The body, electrical, A/C, and Trans are in great shape. The Engine appears to be good also, it just has fuel/power issues I believe. It idles great, but as soon as you press the gas pedal it starts dying. Whether you give it gas or it idles it just stays at 800rpm. I replaced the fuel filter, changed the oil and filter, replaced the spark plugs, the gas pressure regulator, and the fuel injectors. All of this and it didn't really change anything... Still does the same thing. If you leave it to set for a while you can rev it a little bit as soon as you start it, but as it runs and you press the pedal it revs less and less until it's stuck at 800rpm's again. Any insight?