2011 12,1 iMac Drops Wireless (even after new AirPort card)

Hello!

I just recently purchased a 2011 iMac (12,1 that has been upgraded to the 2.8GHz i7 and 8GB RAM). I have had an issue where it will intermittently drop the wireless connection (especially if it goes to sleep, but even if it doesn't) and sometimes it takes a while of waiting because it doesn't see my home network. All of my other macs (and two windows), iPads, iPods, iPhones all see the network and connect and hold the connection beautifully.

I have taken it to Apple and they said something was loose on the AirPort card, so they fixed it and taped it up with something and it should be fine. No go, still happened. Then, I took it back and they replaced the AirPort card entirely. Took three days of waiting, got it back. No go, it is still happening!

What could cause this issue? I use an Airport Extreme that works perfectly fine (and I've unplugged and plugged back in, reset, done everything to that as well) and my internet at home is 25up/25down, so I'm running really quick.

I really do not want to have to take this back to the Apple store -AGAIN- (since it is about an hour drive and three flights up in a huge mall) if there is something I'm missing.

I have it set never to go to sleep, I have it configured to remember the password of the wireless, I have done every single update to it, so, what's the issue?