Wifi suddenly went mad!

Originally I had a 100% functional wifi network including my MacBook Pro (OS X 10.6.8), an Airport Extreme as WLAN router, an Airport Express to stream music from my MacBook, my PS3 and my iPhone, all connected over WLAN.

All of a sudden nothing really works anymore.

Numerous unsuccessfull calls with Apple Care and appointments with Genius bar.

Even bought a brandnew Airport Extreme and requested a brandnew modem from internet provider.

Cables all checked and okay.

Hardware in general seems all okay.

However, WLAN via MacBookPro, etc. is gone. Have tried everything.

Now - I can regain a 100% functional wifi network using my very old Siemens Gigaset SE551 WLAN router, my very old IBM Thinkpad, my fairly new Airport Express, iPhone and PS3!!

But: when I use my MacBook Pro instead of my Thinkpad, the wifi network sometimes works and usually does not. Numerous very strange network phenotypes then.

Wifi radar shows nothing conspicious.

What could I do now??

Buy new Airport Extreme?

Done.

Ask for new modem?

Done.

Cables?

All checked and apparently okay.

Buy new Macbook?

Can't afford.

Does anybody either had the same problem or has a good idea?

Thank you so much!