91 Honda Civix DX

I have a 91 Honda Civic Hatchback DX, 16 valve 4 cylinder engine, fuel injected. Been having starting problems. I literally JUST bought it a couple of days ago and when I test drove it, it started right up. Here recently it's been starting good off and on, but sometimes it just doesn't. It wants to turn over, but it just won't catch. If you push the gas pedal to the floor once and release before you start it, or play with the throttle cable it will start... sometimes. Yesterday, we couldn't get it to start at all and my father in law unplugged something, I don't know what it was, but it was three wires connected to a little yellow piece, and it was connected to the PGM-FI, and it started perfectly a couple of times. I've been told to replace the fuel pump and check fuel injection main relay... Could it be one of those, or what else could it be? It has also been running funny, it has really slow pick up, and  sometimes it will bog down on me going down the road. I have already replaced the O2 sensor and that helped a lot, but it will still do it every once in a while...could that be clogged fuel injectors or fuel pump too? Help please!