Have you installed
CardReset and
CardPatch like yssing suggested? If you haven't, it is the first thing you should go and do right now. Most cards won't work properly on an A1200 and will lock it up until those patches are installed.
Please follow this advice if you haven't already, I have real-life experience with this, my advice is never based on assumptions and guesses! We have several A1200s and A600s set up in our office with various network cards, it's so important to have those patches installed if you don't want to perform a hardware fix!
