what causes this? at say under a load muffler and intermediate pipe start glowing. from the cat forward nothing.

 

from the cat forward, being from the cat towards the front of the car? my best guess would be your car running extremely rich with a clogged cat, but because you say it's after the cat, i'm baffled... it could still have something to do with your car possibly running rich.

 

I'd say that your car is running rich and the cat is NOT plugged. The cat burns raw fuel so all that heat goes into the intermediate pipe and muffler. If the cat is plugged then it will start glowing too since the heat is being trapped inside. But if you run the car long enough the cat WILL start to glow. If it continues to do this the cat will self destruct and WILL get clogged and need replacement. Unfortunately the convertors dont take much excessive heat to destroy them. You will probably need a new cat soon. Better find the cause of the rich condition first or youll burn up another cat.

 

when my cat was full it and the pipe started glowing and then started burning the carpet in the car under the passenger seat.