Slow acceleration at first start up

I have a 1995 Honda Civic DX 1.5 non-vetec.

Every time when I first start up the car in the morning or throughout the day, if the car has been sitting for at least an hour, it accelerates really slow when I press the gas pedal down and I can only go about 20 MPH for about 1-2 minutes, before the car gets going. This has never happened to this car before and I live in Southern California (weather is warm to cool).

Okay, it is that time for preventative maintenance for my car anyways, so I have changed the oil, oil filter, fuel filter, fuel pump, fuel pump strainer, PCV valve, spark plugs, spark plug wires, distributor cap, distributor rotor, and air filter. I have also done my valves and readjusted the timing. After all of this, I thought my problem would solve itself, but I still have the same issue and it is slowly getting worse. I am not upset at all for changing all those items, because it is that time to do so, but I would like to solve my acceleration problem.

Could it be the fuel injectors? I have never changed those since I purchased the car from my father-in-law in 2002. I have only added fuel injector cleaners periodically. Or maybe the fuel pressure regulator?