It may be no consolation but but there's been many reports of abysmal MacPro wireless performance and intermittent stalls. Somewhere there is a post about rearranging antenna connections, and speculation that 10.5.2 has a bug. I'm about 15 feet from my airport extreme and get less signal level (-68dbm at the airport) than an iMac 60 feet and four walls away (-44dbm).
Let us know if you find a fix.