'''for alexank69''':
You're right, the airport is plugged into I/O card. I've already done all things you said: I've reseted the PRAM, reinstalled the OS many and many times and also tried new Snow Leopard OS that I'm still using, also cleaned with compressed air and disconnected and reconnected the airport card and the I/O card too. Now I'm trying new airport update downloaded two days ago and testing if the problem persist. I've also installed iStat to monitor the airport temperature.
There are no special conditions for the airport crash, the length of time is varies each time and the program I'm using when it goes down is different every time.
I've already replaced the airport card and the problem persist. With the first airport card, the mac was fine, maybe till I've done an upgrade, and then the airport stopped working, so I've decided to replace it and with the new card will have the problems described above.
'''for rdklincorporated''':
When I bought the new airport card I've checked if the Apple's seal of approval was intact and it was so I hope that the card was new when I bought it.
I really don't know whether to buy the new I/O card ($100) and try it, cause if the problem persist with this new one card I would have thrown $100 into trash' cause it's difficult to resell this card on eBay or any other site :(
Thank you for answers guys.
PS: Now the airport temperature is 43°C and has just crashed