These are all good suggestions. I would add swapping the "defective" Xircom cards into working pc's that have identical configurations. That way, you'll be able to quickly tell if it's the pc or the card.
A company I worked for in the past had a huge customer base that had both Xircom and 3COM cards in Dell laptops. I helped support them. To be honest, I always received excellent support from Xircom, though I can't say the same about 3COM. Xircom never asked more than 1 or 2 questions before they sent out a replacement. The failure rate wasn't that high either.
Wish I had a number for you, but I think the company had a premier account with a specialized support number...
~cdogg
"The secret to creativity is knowing how to hide your sources."
- A. Einstein