rpms drop check engine light comes on

hello-i have a 1990 bronco2. i just replaced the fuel pump and it ran good for two weeks and now it starts fine, runs well for about 3 minutes, then the rpms drop about 50% and the check engine light comes on. if i put it in neutral, the rpms go back up and the light goes off. when im driving and the rpms cut down, it feels like the gas pedal is harder to push down. please give me an idea what this might be before i take it to a mechanic as i do not have my own mechanic to trust and i just spent $400 putting the fuel pump in...thank u so much~ teri