Why does catalytic converter block?

Vauxhall Astra, 2002, 1.6 ,8V.

The engine light comes on, and when slowing from high speed, the car cuts out. I had the fault read and apparently there was problem with the Cat.
I had it cleaned out, the guys removed A LOT of light grey sooty residue which resolved the problem for a while. Any suggestions what is going wrong?

Neil
April 2010