After trying pretty much everything... New drivers, cloned drive and drastic experiments, etc... NOTHING helped to fix this problem under Snow Leopard. I did see that my Macbook Pro that usually had good test results would do the same thing briefly--So it would show fast, then slow if I *immediately* retested, but even then it sped up a little bit partway through the test.
So, I bit the bullet and upgraded the MacBook Pro to Mountain lion, and that went relatively smoothly. The problem seemed to be fully gone (not that it was very prevalent nor noticeable on that machine).
After a day and a half of sorting the update from Snow Leopard to Mountain Lion 10.8.3 (released just last week) on the Hackintosh, I finally have the system running pretty well.
YES! It appears that the problem is a thing of the past. I now get consistently faster downloads 40Mbps to 60Mbps via tests here. I can browse, and test, and test multiple times in a row and all works fine. I do notice things begin zippier in regular browsing as well.
So, the bottom line is that if you are on Snow Leopard and are having issues with download speeds, your best option may be to upgrade to Lion/Mountain Loin. I really did do about everything I could think of to make this work in SL, or I wouldn't have done the update.
Hope this helps someone not waste all the time on trying to get it working.
THANKS TO ALL WHO SHOWED AN INTEREST AND HAD IDEAS TO SHARE! I doubt I would have done the ML install without hearing that Lion solved it elsewhere.
PS. Re: wireshark... I haven't gotten it running on ML yet since there wasn't a need. Apple support site points to xQuartz for X11 installation. I did that, and didn't get any more messages when starting Wireshark, but it also didn't do anything but sit there in the lower app bar.