Audi Repair: Audi A8 Aftermarket Cats Check Engine Light, o2 sensors, audi a8

Question
I have a 2000 A8 with 150K miles.  The "check engine" light was coming on with codes indicating main catalyst banks 1 and 2 (I believe P0422 and P0432).  I had aftermarket cats put on, but the light came on again after about 50 miles (cleared them a couple of times, but they keep returning).  I am wondering if there is some reason the aftermarket cats trigger the codes and if there is an inexpensive solution.  I've heard somethign about getting the dealer to "flash the ECU" to broaden the cat parameters to "accept" the new cats, but am not sure about this.  The cats from the dealer are too expensive and I am afraid of throwing good money after bad by continuing to chase an aftermarket solution.  What am I missing?  Also, the O2 sensors (all four) have been changed recently, stemming from this issue, but that's another story.

Please help.  Thank you in advance.

Answer
Thank you Graham.  You basically are literally and figuratively the drip that caused the bucket to spill over.  Your feedback is indicative of your generosity.  I hope you saved no money, that the repair will cost you thousands, and also all the other people who may have a question for me, can thank you since they now have to go elsewhere.  I'm done.  There is no pleasing some people, even if it's free.  Why don't you sign up for one of these help lines, you obviously do a great job at criticising other people who try to help for free.  Goodbye
Jan