Firmware?
Hi BOAC,
Have you got the latest firmware in your wireless router (assuming you are going via one) or tha latyest firmaware / drivers for both the Wifi cards?
Are the cards both the same brand? If not they my be using incompatible propriatary tweaks to try and get more speed - fine with like cards, not great with different ones.
Might be worth switching the microwave off too - another 2.4Ghz noise maker - well if it's not totally leak proof.
Memetic