Wifi keeps dropping

I know this is a VERY common problem, and there is already a bunch of info out there, however NONE of that has worked for me and I'm ready to lose my $h!t. So far I have tried...

- Resetting the modem

- Trashing airport preferences

- Talking with apple support over the phone

- wiping the HD and reinstalling the OS

- Replacing the Airport card

None of this has worked. At all. Ever. iPad works fine. PC laptop works fine. Wii works fine. Landlords PC works fine. Landlords Xbox works fine. Friends Macbook Pro works fine. My iMac rejects the wireless signal like it's trying to give it AIDS.

What the **** is going on!?! Apple, why does this VERY common problem persist? Why has nothing been done about it even on the new OS, from what I can see? Are there any other "fixes" out there I haven't tried?