What can I do to restore good wifi reception after upgrading Snow Leopard

Question

My situation is similar to what is described by others, but different enough that I thought starting a new thread might help.  I upgraded from Leopard to Snow Leopard (10.6.3) in January.  All was good.  As new upgrades came out I conscientiously updated my software.  Then about 4-6 weeks ago I noticed that I could not get wireless in some areas of my house, further from my router, where I used to able to get fine reception, and where other computers in the house still worked fine.  The other computers are a MacBook Pro (running 10.6.8 but now just updated to Lion) and an iBook G4 (10.4.11).  I also noticed that when I streamed video it was choppy, even if I used an ethernet cable.
I decided to reinstall from the CD (10.6.3) and that worked; all internet was back to normal.  Then I updated to 10.6.8.  Problem came back immediately. Reinstalled 10.6.3 and all was normal.  Upgraded to 10.6.4 still normal.  Updated to 10.6.5 problem back.  Went back to 10.6.3 and back to normal.
I have tried repairing permissions and some of the other suggestions I have found here, but nothing works.  The only way to get decent wifi reception throughout my house is to go back to the CD and reinstall.   I see lots of people saying that after upgrading to later versions of Snow Leopard they lose wireless, but no one says they just lose wireless at greater distances from the router.  I am using a Westell 327W ADSL modem router (802.11g) and I am connecting to the internet on Verizon DSL. 
I would like to upgrade to Lion but I heard that this does not help with the Wifi issues others have reported, so would like to get this sorted out before upgrading.
Please help.

Answer 1

The solution given by Tenortim fixed my problem.  Thank you Tenortim
https://discussions.apple.com/message/12924009#12924009
It is a little difficult.  You have to be able to use Terminal to do step 3.  Fortunately, my son was able to explain some of the details.  The rest is straightforward. 
I wish Apple would not include the new firmware without making sure it worked.  I suppose if I upgrade to Lion, which I would like to do, I will have to go through the process again.  For now I will stick with 10.6.8 and the Airport driver from 10.6.4, since that is working great.