Strugling to pick up speed. Lack of power!

I have a 1993 4.3 Chevy Astro van manufactured in October 1992 with vin code z. The problem that I'm having is lost of power and is sluggish all time now. A week ago it did not do this. I could accelerate with no problem to pick up speed anytime. The things I started to replace first was spark plugs and wires cap and rotor. I drove my van and still same problem. The next thing I changed was the fuel filter and no changed was noticed. This time I replaced the egr, vacuum solenoid valve and the throttle body injectors. I took my van for a test drive and still no change in performance. So the last thing I could think of is the catalytic converter being plugged up, so I replaced it with a new one. Once again I took my astro van for a test drive and continued with this problem. My van idles great, no back or miss fires. Fuel is 91 octane and flowing good to injectors.I did a visual check for fuel entering throttle body and is okay. No vacuum leaks from intake manifold and hoses. Their is no restriction to the air flow to the throttle body. What other factors can cause this lost of power, bogging down and very sluggish when trying to pick up speed?