Motor Loss of Power

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!

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

Thanks again!

How did you diagnose the Catalytic Converter? My astro just starting behaving the same way. Barely got it off the highway. I was thinking (and hoping) fuel filter, but the Catalytic Converter was the other thing that crossed my mind.

I used a Vacuume guage as pictured below. You can buy one from Auto Parts stores.

http://www.harborfreight.com/cpi/photos/93500-93599/93547.gif
http://www.harborfreight.com/fuel-pump-and-vacuum-tester-93547.html

I removed the driver's side Pre-Cat O2 sensor. I attached the rubber grommet through the O2 sensor hole. If you leave it there too long, it will burn the rubber grommet.
Reading was over 4+psi at idle, and way off the scale (10+psi) at 2000rpm throttle rev.

Based on the "How To Articles," stated the backpressure should not exceed 2.5 psi at 2000rpm rev.

The Cat Canverter is clogged. Made sense. I cut out the CatConverter. The honey comb on input side was broken up in chunks, and the rest sure looked clogged up.