2000 Astro Van
4.3Liter Vortec V6
It used to hesitate and engine code P0300 : multi cylinder missfire.
The the radiator cracked and it over heated. When it started to overheat, also it started to lose power. Not sure if the overheating is connected to the loss of power.
It started to loose power gradually, at this point I can hold the pedal down to the floor and barely get moving. It sounds like it's starving for air or fuel...

Replaced:

Radiator & hoses.
Spark plugs & wires
Distributor Cap & Rotor
Performed Fuel Injection Cleaning.
Checked Cylinder Compression. All 6 are at 200+psi
Checked Fuel Pump Pressure; it's at about 60psi at Ignition On position
Checked if the Intake Manifold gasket is leaking, it's not
Checked vacumme pressure, it seems fine. Air filter is ok.

Does the catalytic converter glow red when this happens. Sounds like it might be plugged....

I don't think that it glows red because it acts the same when the motor is cold or warmed up.

computer read code P0102 Mass Airflow Sensor Low Input. I swapped the MAS and nothing changed.

How to check for clogged CatConverter?

Thanks to everyone who helped out.
It was the clogged Catalytic Converter.
Replaced with a Universal Fit CatConverter.

Thanks again!